Adhesive with a long open time

[Tim Andrews]

Greetings All,
I’ve done a search here, but all the threads I have seen are old. I would like a recommendation for an adhesive with a long open time for complicated glue ups, minimum 30 minutes,. I typically use Titebond 3 but would like more time for some projects. Sometimes it’s possible to do the glue up in sections, but not always.

Titebond Extend has a lot of negative reviews, unless it is now better, so it seems the other options would be hide glue or slow set epoxy. Are these glues as strong as PVA glues? How about dealing with squeeze out? It seems that epoxy would be problematic. Obviously I would want to avoid visible problems when a finish is applied.

Thanks for any advice!

[David Zaret]

i use urea resin for such things. Unibond 800, and i thicken and tint it to get the consistency and color i want. very long open time, but requires some amount of heat to cure - if it's 40 degrees, don't use urea resin. if it hits the 60s or 70s for a few hours, you're fine.

[Michael Burnside]

I use epoxy often. Totalboat with med or slow hardener or System 3 T-88.

Almost every project I build uses epoxy in part of the glue-up. You need a lot less than you think, in fact I think people often apply too much traditional glue too. It's like they forget about long-grain adhesion on wide panels or they forget the tenons are the real strength. <stands on soap box> Glue pouring all over your workpiece isn't "better" or a guarantee that you got adequate glue, it just means you don't know how much was needed in the first place <steps off>. With epoxy you don't need a lot of pressure in clamps so, with practice, you can get a super strong bond with little to no squeeze out. It is also easy to clean up with acetone if you need to. And, like traditional glues, I will often use blue tape around critical seams, inside miters, etc.

Often times glue-ups are complicated due to joinery, so another approach is to glue things in parts. For example, I will often pre-glue my Dominos in place using TB quick-and-thick as it dries pretty fast, letting me get to the next step in about 30-60 minutes.

[Kevin Jenness]

David and Michael offer good advice. Unibond 800 and powdered plastic resin glue are similar urea formaldehyde formulations but Unibond has a lower water content when mixed. Unibond had some reported random problems with staining in the past and it was recommended to mix thoroughly, let rest for 5 minutes and mix again. They may have reformulated to avoid that problem as I have not seen any recent complaints. UF glues have a shelf life so it's a good idea to do a cure test sample on any old stock before committing to a project.

Epoxy is a reliable adhesive with the longest assembly time of any commonly used product. It gradually thickens and can be clamped into place even as it starts to cure, and it makes assembly of mortise/tenon and similar joinery relatively easy as it does not tack and stick like pvas. Curing is an exothermic reaction so it is important to pour out any concentrated volume into a shallow layer to avoid premature cure ("kicking off in the pot"). Although some hardeners tend to darken and stink of ammonia over time the marine epoxies I have used have essentially no shelf life. If the resin crystallizes it can be recovered in a warm water bath. Various additives and colorants can be mixed in to modify epoxy which makes it a versatile product. You can clean up fresh epoxy with vinegar, denatured alcohol or acetone. It does tend to soak into endgrain and darken joints with a visible "hard line". Epoxy bonds best to rough surfaces- smoothly machined joints, especially in dense hardwoods, should be abraded with 80# sandpaper.

I have used Titebond Extend a lot and like it. It has a very stiff cured glueline. It doesn't, though, give that much of an extended assembly time over Titebond l. I avoid Titebond ll Extend as the solids tend to settle out and require periodic remixing.

Urea modified hide glue does give a long assembly time and has the advantage of being repairable, sticking to itself and cleaning up easily with water. I have used only Franklin liquid hide glue which cures slowly with a rather flexible glueline. It is not at all water resistant.

All of these glues are strong enough if used properly. Prefinishing before assembly and using no more glue than necessary can help avoid staining problems.

[Tom M King]

Great post Kevin, but I pulled out this one quote to ask for a comment to see if you worded this as you intended.

" the marine epoxies I have used have essentially no shelf life"

[Kevin Jenness]

Great post Kevin, but I pulled out this one quote to ask for a comment to see if you worded this as you intended.

" the marine epoxies I have used have essentially no shelf life"


Yes, that has been my experience. I have used primarily West System 105/205-6 and Basic No Blush epoxy from Progressive Epoxy Formulators, and have kept them on the shelf for as much as 5 years. I've never had to discard any due to non-curing, although the West System hardeners get pretty dark red and funky.

https://www.westsystem.com/instruction/epoxy-basics/epoxy-shelf-life-proper-storage/#:~:text=WEST%20SYSTEM%20resins%20and%20hardeners% 20do%20not%20have,to%202%20years%20from%20the%20da te%20of%20manufacturing.

[Maurice Mcmurry]

+1 for epoxy. I am most familiar with West System and Smiths. I almost always thicken the glue to paste consistency so glue is not running and dripping. I keep a log with notes on how many grams to mix for various applications. Tape is very helpful. Paste squeeze out can be lifted off with a putty knife and cleaned with denatured alcohol. West System and Gougeon Brothers have published extensively on using epoxy. Their publications are loaded with helpful information and techniques. Working with paste rather than liquid has been a game changer for me. A big thanks to the Gougeon Brothers for all of the educational material over the years.
I also keep liquid hide glue on hand. It is indispensable for things that need to be easily reversible.
I have never had problems with Titebond Extend and use it often.
I recently looked over some exterior doors that were glued with WeldWood Plastic Resin Glue 39 years ago and they still look excellent. I remember the door glue ups being messy.

519619

[andy bessette]

Great post Kevin, but I pulled out this one quote to ask for a comment to see if you worded this as you intended.

" the marine epoxies I have used have essentially no shelf life"

This. Good post. But say "indefinite shelf life".

I have been using WEST epoxy, for wood glue, virtually exclusively for nearly 50 years. Complicated glue-ups are no problem, as there is plenty of working time, as long as you select the correct hardener for ambient conditions (I generally use Fast Hardener).

First, prime all surfaces with neat epoxy mix; plywood edge grain usually requires many coats, wet on wet. If necessary, apply a glue coat of epoxy thickened using colloidal silica (sometimes with microfibers) or high density filler. ASAP remove squeeze-out using, first, a putty knife, then bits of paper towel soaked in acetone. If there is no squeeze-out, you didn't apply a sufficient amount of epoxy.

https://i.postimg.cc/h4YPrTSM/surf-3.jpg (https://postimages.org/)

[Kevin Jenness]

I almost always thicken the glue to paste consistency so glue is not running and dripping... Working with paste rather than liquid has been a game changer for me... A big thanks to the Gougeon Brothers for all of the educational material over the years.


I will point out that West System generally recommends wetting out surfaces with unmodified mixed resin for best adhesion prior to adding thickeners to the mix. I'm sure single step bonding is adequate in most cases but there is some cost in strength. Personally I rarely use thickened epoxy for joinery. I do use thickeners when building up a surface to keep the mix from slumping or to make sanding easier. https://eu.westsystem.com/instruction/epoxy-basics/bonding-and-clamping/

SINGLE-STEP EPOXY BONDINGSingle-step bonding is applying the thickened epoxy directly to both bonding surfaces without first wetting out the surfaces with neat epoxy resin/hardener. We recommend that you thicken the epoxy no more than is necessary to bridge gaps in the joint (the thinner the mixture, the more it can penetrate the surface) and that you do not use this method for highly-loaded joints or for bonding end grain or other porous surfaces.

[Tom M King]

My experience as well, Andy. I haven't found an end to epoxy shelf life yet.

I'll ad another note about a cleaner. I had a boat business back in the 1980's where I sold boats and worked on them. One day one of the young guys working for me spilled some epoxy hardener on the concrete floor back in the shop. About that same time a salesman showed up at the door selling some new type of orange oil cleaner that he claimed would clean anything. It was a concentrate that he sold in gallons.

I told him that if it would get that epoxy hardener, which was a decent sized puddle, off the concrete floor that I would buy a case of it. He grew a big smile and ran back to his truck. It not only took the hardener right up, but cleaned the concrete floor down to look like it was finished last week and didn't take long to do it.

That case of gallons of that cleaner lasted me for a couple of decades. They sell similar things these days, but I've never seen the exact thing again. This stuff came out of Florida.

[Tom M King]

One question: What is neat epoxy resin hardener? I just don't understand what that means.

edited to add: As a correction to my earlier comment on epoxy, I should add a little clarification. Golf epoxy does have a shelf life of a few years. I don't know the differences other than it's engineered to break down at a temperature about a hundred degrees less than epoxy used in the making of composite golf shafts. It's probably heat that shortens its shelf life I expect.

[Kevin Jenness]

This. Good post. But say "indefinite shelf life".

Agreed. For my practical purposes there's not much difference. For highly stressed joints newer stock may be advisable. https://www.epoxyworks.com/index.php/shelf-life-real-life/From the West System link in my earlier post, " We have mixed 15-year-old WEST SYSTEM 105 Epoxy resin with a newer hardener and it cured fine."

[Kevin Jenness]

One question: What is neat epoxy resin hardener? I just don't understand what that means.

It means mixed resin and hardener with no additives.

[Tom M King]

Thanks....

Another note for those new to epoxy: When even opening the top of colloidal silica thickener containers, wear breathing protection. The stuff is Very fine and floats in the air. You don't want to breathe it. When silica gets in your lungs, it doesn't ever come out.

[Michael Burnside]

I will add that when I use epoxy I mix in small batches. In small paper cups. It’s so easy to mix, takes 60 seconds to mix two part epoxy and I’m ready to go. It’s expensive so I don’t want to waste it but, again, goes a long way and since it has a longer open time when using medium/long hardeners, there is less stress making more.

[Jim Becker]

I'd use T-88 epoxy for this situation...that's actually why I originally bought it: a complicated glue-up that was going to take time to get "just right".

[stephen thomas]

Random notes:
The problem with urea glues is that they have about a 20 year half-life in service. Urea-formaldehyde resins self-destruct over time, more so with increased heat.

WEST epoxy addressed the concern about darkening/thickening hardners over extended storage life a few decades ago and found that the older products actually produced slightly stronger bonds. The staining can be an issue, though.

I used the original Titebond extend, which is a great glue. Originally it had great open time, and was the most rigid, strongest developed strength joint of the entire Titebond line-up They changed the formula (acknowledged) - i forget what it corresponds to now. Still use it, but not quite the same. It's now not much more open time than the original but i think it is a slightly stronger version.

I've been using WEST since the late 70's/early 80's. Some years 2 or 3 C units (5gal units) Then sometimes there would be a 5 or 6 year span where a C unit would last the entire time for only small projects. Never a problem with use. I think many of us were reluctant to move to much cheaper products as they came out, because WEST is so reliable/predictable, and has a good system for dispensers, hardeners and engineered fillers.

So long as WEST is mixed well in a round smooth-sided container, then spread out into, say, a paint roller pan, it will last at least the predicted open time for the temperature and hardener chosen. If it is spread quickly on all mating parts, it can allow well over an hour assembly time before all the clamps need to be on. For complex jointery, it is nice because things go together like they are greased. Unlike Titebond or others that can grab and seize up with a delayed attempt to clamp.

519599

[Kevin Jenness]

Random notes:
The problem with urea glues is that they have about a 20 year half-life in service. Urea-formaldehyde resins self-destruct over time, more so with increased heat.

Please expand on this. I can believe that the rigid glueline could be a disadvantage in some situations like crossgrain joints subject to moisture cycling. In his book Professional Cabinetmaking Alan Peters mentioned a UF failure in mortise and tenon chair joints and his preference for more flexible pvas. I would think though, given the widespread use of uf adhesives in veneer work and plywood manufacturing that this would be a widely known and disqualifying issue. Can you provide some documentation?

[Maurice Mcmurry]

No problem with Urea-formaldehyde (so far) on these 39 year old exterior doors.

519600 519602 519603 519604

[Alan Lightstone]

I just purchased some West Systems 209 Extra Slow Hardener. The web site says: Pot life of 40-50 minutes at 72°F, and working time of 3-4 hours. Haven't used it yet, but should give me plenty of time for my complicated glue-up.

[andy bessette]

Please expand on this. I can believe that the rigid glueline could be a disadvantage in some situations like crossgrain joints subject to moisture cycling. In his book Professional Cabinetmaking Alan Peters mentioned a UF failure in mortise and tenon chair joints and his preference for more flexible pvas. I would think though, given the widespread use of uf adhesives in veneer work and plywood manufacturing that this would be a widely known and disqualifying issue. Can you provide some documentation?

I have seen instances of its failure in much less than 20 years, where it has literally turned into powder, with no adhesion whatsoever. These issues were all seen where it had been used by other boat builders.

[Tim Andrews]

OP here, just checking in after posting my question last night. Wow, thanks for all the great replies! This has been a very informative discussion. I want to give epoxy a try, of course I’ll make some test joints first.

I haven’t searched yet, I’m assuming the better products like West or Totalboat must be ordered online as opposed to being available at the local Borg? And is my understanding correct that some epoxies are for adhesion, and others are for pouring into forms for projects like tabletops? Thanks again.

[Kevin Jenness]

I have seen instances of its failure in much less than 20 years, where it has literally turned into powder, with no adhesion whatsoever. These issues were all seen where it had been used by other boat builders.

To be fair, uf adhesives are not marketed as waterproof or for boatbuilding uses exposed to severe moisture cycling. Can you describe the typical uf failure scenarios you have seen? Have you seen that sort of problem in more typical interior environments?

edit: I did find this report from the FPL about moisture/heat degradation of uf bonds. https://www.fpl.fs.usda.gov/documnts/pdf1994/river94c.pdf

[Kevin Jenness]

[QUOTE=Tim Andrews;3315590I haven’t searched yet, I’m assuming the better products like West or Totalboat must be ordered online as opposed to being available at the local Borg? And is my understanding correct that some epoxies are for adhesion, and others are for pouring into forms for projects like tabletops? Thanks again.[/QUOTE]

West System products can be found fairly widely at suppliers like West Marine, but generally your best bet is online. There are a lot of specialized formulations for clear coating and deep pours/casting so do your homework.

[Maurice Mcmurry]

Jamestown Distributors has been my source. The slowest version I have tried is the "Tropical" hardener. It is very slow.

https://www.jamestowndistributors.com/product?page=1&resultsPerPage=16&filterBy=%26filter.g_brand:WEST%2520System&gad_source=1&gclid=CjwKCAjwrvyxBhAbEiwAEg_KgoPH_v0mJH_zjJMjXx8B 7kH9EhmVNrw4cBpHHO9DDu4mKlSHd98PaxoC__gQAvD_BwE

[John Kananis]

I like both west systems epoxy and unibond 800 for longer assembly time.

That said, I don't think anyone has asked what application this is for? And oftentimes, you can use standard pva glue if you glue in sections. Assemble the entire project and then glue the last element in place. When that sets, remove a piece, apply glue and put back in the clamps, rinse and repeat until all the joints are glued - this method also guarantees that everything fits together properly.

[Tim Andrews]

Thanks again for the recommendations. Unless I’m using exposed joinery like dovetails or box joints, I usually use my Dowelmax because I like the speed, strength, and accuracy it provides. For a larger piece, that can add up to a lot of dowels. I understand the process of gluing in sections; dry fitting the entire piece, disassembling a section, applying glue to only that section, and then reassembling and applying clamps, rinse and repeat. Just want to explore the options that a longer open time offers.

[stephen thomas]

Please expand on this. I can believe that the rigid glueline could be a disadvantage in some situations like crossgrain joints subject to moisture cycling. In his book Professional Cabinetmaking Alan Peters mentioned a UF failure in mortise and tenon chair joints and his preference for more flexible pvas. I would think though, given the widespread use of uf adhesives in veneer work and plywood manufacturing that this would be a widely known and disqualifying issue. Can you provide some documentation?


Please expand on this. I can believe that the rigid glueline could be a disadvantage in some situations like crossgrain joints subject to moisture cycling. In his book Professional Cabinetmaking Alan Peters mentioned a UF failure in mortise and tenon chair joints and his preference for more flexible pvas. I would think though, given the widespread use of uf adhesives in veneer work and plywood manufacturing that this would be a widely known and disqualifying issue. Can you provide some documentation?

When i started woodworking as a kid in the late 50's/early 60's, about the only glues out there for "consumers" were liquid hide glue, and Elmers.
Elmers did not work well in my then limited experience (I recognized later that it does have uses) and hide glues failed when they got wet, especially the "liquid" versions.
I remember taking a (small) creche i made in 4th or 5th grade woodworking/craft class for the school display home after Christmas. It was a long bus ride on a very wet, rainy day, and the parts were shifting after the first bus transfer. By the time i hiked up our lane, it was just a handful of molasses coated sticks. By HS i was exposed to Weldwood plastic resin glue, but did not have good luck with it (my learning curve, not the product. Possibly combined with outdated glues in the local sources, or storage.) Later in my working life i was exposed to all sorts of glued wooden assemblies. I was struck at some point how many failed assemblies and delaminations were apparently initially stuck together with Urea glues. That had turned to powder.

When i got older and started woodworking professionally, i made a point of researching the options as much as could be found in books Remember, you could not just go online and search the FPL in those days.
In 1978, "Adhesive Bonding of Wood" M.L Selbo came out and essentially was a compendium of various FPL papers and experiments of a range of glues in a range of wood substrates, over all kinds of conditions and time spans.
Regarding Urea resins, it became apparent how easy it would be to not use urea glues optimally. In tests, yes, the glue, substrate, and environment really do need to be above 70F before and while being pressed, the press time really does have to be in tons, and the duration of pressing really does have to be "long". There are many ways to go wrong just in the prep stage, and while prepping to clamp (very limited open time). In an industrial setting when separate stronger acid catalysts are used, the opportunity for mayhem in later years increases. (While most small shops do not use separate acid catalysts, it is mentioned as yet another possible gremlin that may have occurred when inspecting a failed manufactured item.) Apparently, the room setting formulations are the least durable, except only for those that are extended (say, with wood flour).

The section in ABoW about urea durability is almost an attempt at an apology, and describes all the ways that if it was perfectly pressed, and perfect environment, it will generally be pretty good, maybe even excellent for fortified types. (not room temp curing) After working down through extreme temperatures for failures that a typical item is not normally exposed to, it eventually notes that "...less significant weakening of room-temperature-setting urea resin glue joints has been observed in birch plywood under continuous exposure to 80F & 65% RH. The rate of strength loss is increased by high humidity at 80F" & above. Kind of sounds like most of central and eastern US over a long summer for high humidity, and the whole country for extended periods above 80F Unfinished specimens can deteriorate faster. A summary includes "Nevertheless high-quality urea resin glue joints do appear to be sufficiently durable for nonstructural interior applications within the human comfort range of temperature and humidity conditions. On the other hand particularly with high shrinkage, dense species, the more durable resin adhesives {referring to phenol resorcinal and melamine adhesive, e.g.} would assure longer trouble free life"

When i started messing about with small airplanes, "plastic resin" glues were approved for primary structure. Sometime in the past few decades, they became prohibited. While an airplane is very well likely to be exposed to wider temp and humidity conditions, routinely, than, say a credenza, the stark fact remains that the glue turns to a powder and fails. My experience with inspecting or sometimes repairing other manufactured items; and i believe i read this somewhere, is that about half the remaining joints fail on about a 20 year basis.

Hot hide glue has a better service history. The real factor for me is that it can be repaired when it does fail, so i use it for traditional repairs and some assemblies.
Titebond (family) works for mechanical joint assemblies that are not exposed to outdoor conditions.
Phenol Resorcinol is for exterior intended to last "forever" assemblies especially laminations.
Epoxy is for "nearly forever under "most" all conditions where the wood won't fail first. And for when i need a very long assembly time.

The deterioration rates and weaknesses of the glues i do use are known and predictable.
Urea resin seems a whole lot less so, and it offers no advantage to me that is not better served by one of the others.

It is not important for me to "convert" anyone. There could certainly be reason in many peoples work to use room-temp-setting Urea glues.
If you have reason besides my experience and caution, the FPL can now be researched online. If you find counter information as part of a complete analysis, please post it.

smt

[Mark Hennebury]

Thanks Stephen.

[Kevin Jenness]

Stephen, thanks for the explanation. What experience I have (relatively little) with room temperature setting uf glues has been veneering interior millwork which has not been exposed to extreme conditions and as far as I know has held up ok. I agree with your assessment of other classes of adhesives and am not an advocate of uf glues in general but I will be even more skeptical in future thanks to your post.

[Tom M King]

Restaurant suppliers and fiberglass supply stores sell sleeves of wax free paper cups. Probably on eBay too.

[Michael Burnside]

Restaurant suppliers and fiberglass supply stores sell sleeves of wax free paper cups. Probably on eBay too.

Yep I got mine from a restaurant supplier on Amazon I think. I have 500 little wax-free paper cups which are awesome for most projects.

[Don Peters]

Cat food cans; you've already paid for them. And you can often pop any hardened residue out and recycle them.

[Wes Grass]

I glued something up with UF. Waited until a warm part of the year. Santa Cruz mountains... 'don't like the weather ... give it a minute, it'll change' ... McMaster ... next day a small space heater and a couple heavy moving blankets I already had ...

House didn't burn down while I was at work...

[Wes Grass]

Cat food cans; you've already paid for them. And you can often pop any hardened residue out and recycle them.

The only 'hardened residue' I find is cat food ... our 2 'boo's' leave behind. One is a picky little be-atch, showed up at the door one night ...the other a psycho polydactyl 'Highland Curl' ... bred in Texas.

[Randy Heinemann]

I’ve never had a problem with Titebond II Extend. However I doubt it would give you. 30-minute open time. I’d say no longer than 20 minutes and that might be pushing it. Epoxy is most likely the adhesive to use for 30 minutes open time.

[Michael Burnside]

TB II extend is my go to as well Randy. But yea, doesn’t even compare to open time of epoxy with a slow hardener.

[Mike Henderson]

One thing to watch for when using epoxy with a slow hardener. The chemical reaction generates heat and heat speeds up the reaction. If I mix up a fair amount of epoxy for a large glue-up, I get some ice and put my container of epoxy in the ice to keep it cool.

The first time I used a significant amount of epoxy - without ice - it hardened up on me in the container before I was finished with the glue-up.

This was West Systems with the slow hardener and maybe two squirts of resin and hardener.

Mike

[Kevin Jenness]

There's always a risk of premature curing. Ice and/or a fan can help dissipate heat from the mix but the first lines of defense are to mix in small batches and get the mix spread out quickly in a flat pan or on the work to increase the surface area. I had a batch kick off in a roller pan once to the point that it started smoking and I had to toss it out a second story window for fear of ignition. Once you get a whiff of the devil's throne room coming off a hot batch of epoxy you won't want to repeat the experience.

From an Epoxyworks (West System) post: Epoxy heating out of control can foam, smoke, give off dangerous vapors and generate enough heat to melt its container or cause nearby items to catch fire. https://www.epoxyworks.com/index.php/controlling-exotherm/

[Tom M King]

I had a boat business back in the 1980's that included repairing fiberglass and bulding sailboards using both epoxy and polyester resins. I had an oversized air conditioner that could cool the shop down to low 60's fairly quickly for layup, then we'd turn it up to normal to let it cure. It didn't matter how hot it was outside and worked fantastically.

I expect most of us have done what Kevin was describing of making epoxy foam. A local guy told me he was doing some auto body work in his yard. He mixed up a gallon of Bondo, and went in the house to eat lunch before going to work with it.............

[Maurice Mcmurry]

Allegedly Robert Manry was one of the first boating enthusiasts to prove the usefulness of epoxy for wooden boat building and repair. I have posted about Tinkerbelle several times. It is an interesting story.

https://www.robertmanryproject.com/tinkerbelle/

My first experience with epoxy was helping my brother create air tight waterproof foot lockers for storing and transporting 54 inch commercial wall covering. His pasting machine was permanently set up in his shop and he would show up at a paperhanging job with the wallpaper all pasted and ready to go, stored in his custom airtight foot lockers. His main interest was cars and his first attempts to waterproof his wallpaper boxes were with polyester resin and fiberglass. Polyester did not work. West System solved the problem. Back then you had to call the Gougeon Brothers on the land line to get advice and order resins. With the leftover epoxy from the waterproof footlocker project I built a boat.

519703

[Prashun Patel]

I've used System 3 T-88. It has about 45 minutes open time. I recommend it.

However, you mentioned "complicated" glue up. Beware that 2 part epoxies require mixing, and are high viscosity, so they are slow and messy to apply (you can't really brush it on that easily as you can Titebond).

So for complicated glue ups, you should consider using a combination of TB for some of the joints and epoxy for only the ones that can't be quickly clamped.

[Jimmy Harris]

You can push the open time of PVA glues a bit by adding a little bit of water and using them in colder temperatures. It also helps if you can raise the humidity or refrigerate the glue (don't freeze it). I've pushed TB3 to about 30 minutes and had no issues doing that. PVA hardens by evaporation, so if you slow the evaporation, you slow the drying speed. Just don't dilute the glue too much, where it gets all runny and thins out on your joints. The idea is to add the same amount of water to the glue that will evaporate while it's open, so it'll have the same water percentage in the glue when you go to clamp up your work as fresh glue out of the bottle would have.

A long set epoxy would be the better option, but those are some tricks to try to get a little more open time out of PVA glues, should you ever need them.