Epoxy fill for cracks and voids

[Rob Luter]

For visibility, I'll also cross posted this on the finishing forum. Hoping the mods forgive...... I'll delete one if need be.

I'm working on a piece that will need some large cracks and knotholes filled with epoxy. I've done some of this on small voids and cracks with 5 minute clear two part DEVCON from the hardware store and it's worked exceptionally well. This project will require more volume, perhaps four or five fluid ounces. I also need something with a thinner viscosity so it will fill deep narrow cracks. All I can find at the local woodworking store is two part bar top finish that based on the price contains gold or platinum. The home center has the same sort of stuff but at a more down to earth price. That said, neither claimed they were suitable for filling voids or cavities.

What say the Sawmill Creek Illuminati? Does anyone have experience with something that doesn't set up too quick, will take dye/stain well, and is available in a smaller (pint) size? I'll also prefer it not be too spendy but I'll pay whatever I need to for something that will work.

Thanks in advance.

[Paul F Franklin]

Take a look at system 3 T88. Available in various sizes, takes pigment well, and has a working time of 30 minutes or so. Fills voids well; not sure about deep, narrow cracks though. I don't think it's any more viscous than the DEVCON though.

[Lisa Starr]

I've used West Systems Resin 105 with 205 Hardener for jobs like that. The really thin viscosity seeps into the fine cracks easily and I've colored it with Transtint
Dye.

[Stan Calow]

I started using the System Three General Purpose Epoxy. You can mix it with fast, slow or medium hardener. It is very liquid, in fact you might be tempted to think you need more hardener but it hardens just fine and pretty clear (slight yellow tone). I use T-88 all the time for structural things, and don't think its good for this because it is very thick when mixed (like vaseline)and doesnt flow well.

I am surprised that you didn't find this in your woodworking store but maybe you didnt look in the glue section instead of finishing. I think I've seen it (or West - another good product) at Lowes Depot.

[Rob Luter]

@Stan - My local woodworking store has glue and finishing in the same area. After further research, it seems the bar top finish they had would probably have worked, but it would have been cheaper to pack the holes and cracks with gold. Lowe's had some similar self leveling finish, but it was one part and the usage guidelines didn't predict success. I found some suitable two part stuff (https://www.alumilite.com/resins/amazing-clear-cast/?gclid=CjwKCAiAxJSPBhAoEiwAeO_fP2Ter_Rwwaaa72rtSE9 DpwVFDhYyOOo57Rv83WyC3j2EnBBv7rta_BoCfU0QAvD_BwE) at the local craft store (image below) and it was reasonably priced. They had a number of different sizes available too, from 4oz to a gallon. I mixed a couple ounces and dyed it with some India Ink. It's thin enough that it did a great job of filling the voids and wicking into the cracks.



https://cdn11.bigcommerce.com/s-2yiip5r05b/images/stencil/1280x1280/products/237/952/ACC_8_oz-2__33687.1636577670.jpg?c=1

[Kevin Jenness]

There are hundreds of resin formulations that can be used for this application including polyester, epoxy and polyurethane. For small quantities you are probably best off going to a craft store and looking for casting resin. Whatever you use follow the mixing directions precisely and pay attention to the working and curing times and temperatures.

I buy marine epoxy in gallon quantities as it is quite versatile and has a very long shelf life, so that is what I have the most experience with. Most such formulations have a low enough viscosity to fill small cracks and voids and a reasonably long working time. The main problems I have encountered with epoxy fills are bubbles, bleeding and cracking. I believe these are common with most resins.

Bubbles are typically the biggest problem and come from air entrained by mixing and air emerging from the wood or trapped by the pouring process. They will slowly rise toward the surface, faster with a lower viscosity mix. If the resin cures before they emerge you will get a cloudy fill or holes in the finished surface. This can be minimized by overfilling the voids (needs a dam of hotmelt glue, caulk, etc.), warming the substrate and/or resin before filling and lightly heating the surface with a heatgun or torch. Adding heat hastens the cure and decreases working time so you can easily overdo it. Small projects can be cured in a pressure pot which will compress the bubbles to invisibility.

Bleeding can mean leaking out through unseen cracks or staining the surrounding wood, particularly endgrain or punky material and with pigmented resin. The first is somewhat unpredictable and about all you can do is try to seal off any escape routes with tape and gub. The latter can be decreased by pre-sealing the walls with a light coat of clear resin or ca glue.


Cracking can come from too fast curing of an overly large or deep pour. Epoxy resins cure in an exothermic reaction so the larger and better insulated the mass the more heat is generated. If you are filling a deep cavity you may want to work in lifts, the maximum depth depending on the specific resin.

I just got some casting resin called Liquid Diamonds that is very low viscosity and said to cure bubble free without pressure. https://www.turnerswarehouse.com/products/liquid-diamonds-epoxy-casting-resin?variant=42280872844 I haven't used it yet, but I have hopes.

[Scott Winners]

I too am a fan of West Systems. Very long shelf life as Kevin already pointed out. Available in various sizes. I buy the 105 resin by the gallon and, lemme go look, I buy the 207 special clear hardener by the half gallon but also currently own 205 (fast) and 209 (slow). 205 and 209 will have a slight yellow tint once fully cured. I like some pretty clear finished wood on every boat I build, so I kinda default to 207 but use fast/ slow as needed depending on what is getting glued up.

This is not a product that is going to get mastered in 6-12 months. Competency is only 2-3 glue-ups away if you find a good teacher, but mastery is a slow process. IIRC I took my first boat building class about 7 years ago and consider my self an advanced novice - and I have only used the 105 resin and the 3 hardeners above.

Couple concerns. I cringe when I see folks using West 105/2xx as a crack filler without knowing their local humidity swings. You can certainly call then, Geugon brothers I think is the parent company in Bay City Michigan. If you are going to buy something from them they want you to be happy with it. In this instance I think you want something that will cure pretty firm, but not act as a wedge and split your project up if it gets exposed to some humidity, so hot hard like a diamond.

Somewhere near you, hopefully, is an amatuer boat builder or 20. I have never yet met a boat builder who wouldn't talk my leg off if I asked for advice, and volunteer to come over to my shop to help out. Some of 'us' are a little squirrely, so maybe choose wisely, but overall a friendly bunch.

Also as Kevin already mentioned I wouldn't use 105/20x for a thick poor. Given the lamentably long life span of the live edge river pour fad I am sure there are a number of products that can meet your needs, from many manufacturers. Epoxy is a rabbit hole, but it is less expensive than lusting after a lathe.

Like Kevin I tend to heat my substrate before applying epoxy and have a pretty good feel for how hot I can get the epoxy with a heat gun after it is applied to the substrate without causing bubbles. Takes practice, and I sometimes still overshoot. Epoxy is great stuff, I have never bought a bottle of TBIII; but it is not a magic bullet.

[Rob Luter]

Kevin and Scott - Thanks for the insights. I see how this could be a real rabbit hole. Thankfully for now I'm just using it to fill knot holes and cracks and I expect that's all I'll ever do.

[Jim Becker]

One thing that is clear to me from watching many others do this, along with my own efforts, is that syringes are indispensable for this process, both straight and curved. It's a lot easier to target things, especially the smaller cracks and voids and it also reduces sanding labor when using tinted filler as there is less surface staining around the fill site.

[Mark Gibney]

I like SculpWood paste for filling cracks and checks.
You can blacken it with dye or with powder, and it squishes into cracks easily with a putty knife.
It has about a 30 minute open time.

The best thing is that it sands easily, but is plenty strong enough for everyday wear and tear. Sanding epoxy can be difficult for me - the surrounding wood can end up being sanded more than the epoxy.

[Jim Tobias]

Rob,
I've used the Alumilite Amazing Clear Cast several times and like the results on thin pours or shallow fills. It cures quick and is clear (unless color added). It is only good for up to 3/8" unless you do the 3/8" and come back with additional pours. I don't know how deep your knot was but for future reference, Alumilite has other expoxy resins that can go deeper. Not affiliated, just a satisfied customer.
Jim

[Rob Luter]

Rob,
I've used the Alumilite Amazing Clear Cast several times and like the results on thin pours or shallow fills. It cures quick and is clear (unless color added). It is only good for up to 3/8" unless you do the 3/8" and come back with additional pours. I don't know how deep your knot was but for future reference, Alumilite has other expoxy resins that can go deeper. Not affiliated, just a satisfied customer.
Jim

Thanks for the input Jim. My pour was deeper than 3/8" so I'm taking it in two stages. I also wanted to make sure I got the surface finish results I expected (I did). I need to find some disposable syringes for some of the cracks. Either that or do some very creative masking with tape.

[Dean Egnater]

I've used both System 3 and West systems...both worked well. I actually used T88 for project below and that worked as well (and added structure) I used syringes but still find I need to follow up with something to poke the epoxy down into the cracks with. Also, I used to use tint/oil paint to color the epoxy and but I found out you can use black graphite instead. Cheap and super easy...and it doesn't stain the surrounding wood.

You can see a project I did with a super punky checked board....and around half a gallon of epoxy here.... https://sawmillcreek.org/attachment.php?attachmentid=441228&d=1600289383