Best Gluing Practice with Shop Sawn Veneer?

[John TenEyck]

I make 1/16" shop sawn veneer, assemble panels in the usual with with veneer tape, and then glue it onto plywood, MDF, or particle board cores in a vacuum bag. By the time the panels are finish sanded I would estimate that the veneer is 0.050 - 0.060" thick. I've used Weldwood Plastic Resin Glue almost exclusively for at least years, but I also have used epoxy when I glued some to one side of Melamine after sanding off the coating. The MC of both the veneer and cores has always been between about 6% and 9% (30 - 55% RH), Winter to Summer in my heated, dehumidified basement shop. The temp. has always been at least 70F and I leave the panels in the press for 13 - 14 hours at roughly 20" Hg.

About 3 years ago I built a vanity for my own house, using ash over the sanded Melamine with epoxy, and on MDF with PRG. After a year I noticed one of the veneer seams on the Melamine panel had split opened a little. That was in the middle of Winter. The next Summer I couldn't see it. The next Winter I could see it again, plus a couple more. This Winter I see a split seam on the two doors of the cabinet, which were done with PRG on MDF. All of these panels are about 32" tall with the veneer running horizontally, so 32" of cross grain if you will. My house gets down to the same 30% RH in the Winter as my shop, and goes as high as 75% sometimes in the Summer unless the AC is on.

None of the seams on that cabinet were preglued when I taped up the veneer panels. I built a storage cabinet for the same bathroom 2 years ago, using more ash veneer over MDF with PRG. On those panels I did preglue the seams with TB II or III when I made the veneer panels. No seams have split open on those doors.

Last year I started rehabbing my kitchen using, you guessed it, more ash veneer, this time glued with PRG on composite panels - plywood core with particle board on both sides. I preglued all of the seams with TB II on the veneer panels before gluing them to the cores. Earlier this week I noticed a few of those seams starting to split.

So I've seen seam splits on particle board and MDF cores glued with both epoxy and PRG. I don't recall ever seeing a split seam with plywood cores, thankfully, because I have quite a few customer projects out there. I'm sure I would have heard; who knows, maybe the phone will ring any day. Most of the projects I've sold have used maple, white oak, and walnut veneer; none with ash.

I'm working on another section of my kitchen rehab this Winter using the same materials and process, except for a few panels that I substituted poplar plywood for the substrate just for comparison. I have always used the recommended spread rate for PRG of 1 lb of resin per 40 sq. ft of substrate, regardless of which type, but after seeing the seam splits I've increased it to 1 lb/35 sq. ft and it's noticeably heavier when I roll it onto the substrate. Of course I won't know if this helps or not for about a year.

This has been a real blow to my confidence in using shop sawn veneer, with real concerns for the projects I've sold. I'm looking to see what others are doing, what your track record has been, what you think I might be doing wrong, etc.

Thanks in advance.

John

[Mel Fulks]

Do you put glue on both surfaces ? I was taught to do that and I think it works better than one heavier coat. I also think it does better if you let the glue rest at least 10 minutes after mixing before using it . It used to say that in the directions,
I think they took it off just in case buyers might find that an unnecessary complication and just buy something else. We
always mixed it to where it dripped ,not streamed ,off stir stick before its 10 minute rest. John, I'm sure you've seen all
this before. But I would not dare to change the method I was taught.

[John TenEyck]

Thanks Mel. No, I only put glue on the substrate, not the veneer. The directions I downloaded 10 years ago or so caution you not to put glue on the veneer because it will curl, and just putting the panel with glue on it will cause it to do so if you don't work quickly. I have always used the glue immediately after mixing, but I recently wondered if letting it sit for 5 or 10 minutes might be beneficial. Below is from Weldwood's current website:


PREPARATION 1. Surfaces should be clean, dry, smooth, and free from all foreign material. 2. Water temperature must be between 68°F (20°C) and 100°F (38°C). Higher water temperature will result in shorter working time. 3. Measure powder and water into separate containers. 4. Add powder slowly to the water while mixing continuously until uniform mix is achieved. Final consistency should be a smooth, creamy paste. 5. If mixture does not combine to smooth consistency, or mixture results in a gritty or lumpy texture, product may no longer be suitable for use.

APPLICATION: 1. Apply a thin coat of glue mixture. Application of glue on only one of the bonding surfaces is generally sufficient. Use a paint brush, paint roller, 1/32” notched trowel, spray gun or conventional glue spreader. 2. Clamp or screw bonded materials together with sufficient pressure forcing excess glue from joins to release trapped air bubbles and ensure maximum adhesive coverage between surfaces. Refer to table below for recommended clamp times. 3. Clamping Pressure Required: 50 PSI for softwood or well machined hardwood; 175 PSI for rough cut softwood materials; 300 PSI for rough cut hardwoods. Finished glue line thickness should be from 0.003 + 0.006 inch for best results.

Still nothing about mix time/rest time. But now they say glue on one side "is generally sufficient" which means sometimes it's not. It's still not practical to apply it to the veneer, IMO, so using more glue might be the correct solution. I've wondered about glue spread rate before since a vacuum bag applies less than 10% of recommended pressures.

I have done some peel testing on samples. Typically the bond is so good that the veneer chips off taking pieces of the substrate with it. But once I was struggling to get the parts under vacuum before exceeding the open time. After curing I could see the bond was poor along the edges so I tried pulling it off. Some came off in full length strips with just sporadic coverage of glue, while some was well bonded and pulled up the substrate.

John

[Jamie Buxton]

The plastic resin glue has water in it. Perhaps that water is making the veneer wider while you’re gluing it down. When the water eventually evaporates, the veneer is trying to shrink back to its original width. To test my guess, measure the width of the sheet of veneer before and after the gluing operation.

Me, I glue sawn veneer with epoxy, because I’m leery of water in the glue. I use West Systems. I use plywood for the substrate. So far I haven’t seen seam failures. But I have admit that the humidity swings here are narrower than in upstate New York.

[Mel Fulks]

With the 10 minute rest the stuff does act diferently ,it no longer beads up requiring more brushing for even and complete coverage. It's possible that the small gap that opened was caused by a little moisture making "compression ring
set" in normal use.I think glue between the pieces is needed,certainly I concede that getting glue there is almost unavoidable
I mention it only because if you wanted to avoid it I'm sure you could do so. I'm sure you will figure out a small new
procedure that works and I look forward to reading it.

[Jamie Buxton]

I should mention that I don't edgeglue the veneer leafs together. My reasoning is that if I do, I might get lippage. Then, when I glue the veneer to the substrate, I'd have an air gap at one side of the seam. So I just veneer-tape the leafs together, and glue them down.

[johnny means]

Hate to ask, but are you putting your veneer tape on the exterior side of your veneer? I've seen tape under the veneer be a point of failure. The tape splits when subjected to sheering force.

[Jeff Duncan]

I do a lot of stuff with shop sawn veneers and have always used plain old yellow glue. If the pieces are thick enough, like your 1/16" pieces, I edge glue them as well. I don't generally use veneer tape, I'll just use blue tape on the outer faces to keep them tight while I get them into the bag.

Outside of your gluing practice you may also want to consider the humidity aspect of your situation. If your home humidity is down to 30% I'm guessing your not providing any humidification to your home in the winter? This is going to cause everything wood to completely dry out and shrink. I myself keep 3 humidifiers going pumping about 4+ gallons of water back into the air daily this time of year for an apartment under 1000 sf. Even then it can be tough to keep the relative humidity at a decent range, but its WAY better than not humidifying at all.

just something to think about,
JeffD

[John TenEyck]

Thanks everyone for the added input.

Mel, you may be on to something. I notice that the glue tends to sort of fish eye when I first roll it out, using a foam roller BTW, but after a minute or two I roll it again and it lays flatter, like finish does when it flows out. I'm going to try letting it sit in the pot for 5 or 10 minutes next time before spreading it. And I've got a couple of seams that look funny but aren't split, at least not yet. They look funny in that the dye toner doesn't looks a little lighter on an 1/8" on each side of the seam. No one else but me sees it, but it almost looks like when you get yellow glue on wood and don't get it all off. Maybe this is compression ring set you mentioned. Or maybe it's just the quarter sawn character at the edge of those pieces being less absorbent or reflecting light differently. Other panels made in the same sequence in the same way look perfectly normal.

I'll post some picture tomorrow, but tonight I split apart a couple of panels I had put maple veneer on by mistake instead of ash. The veneer tore the particle board substrate apart, including right up to the veneer seams.

Where the hardwood edging was, however, it split cleanly along the glue line. That was not encouraging, but I've never had a problem with any panels over time. And

I saw no glue between the seams, which there actually should not be if the seams stay tight. So if you need glue between the seams you need to preglue them or use a glue that better absorbs into the seams.

Jamie, I stopped edge gluing for the same reason you mentioned, edge misalignment. I think the cracks I'm seeing on one panel might be due to that problem with some veneer that badly misbehaved.

Johnny, I've made a lot of mistakes over the years, but I've never put the tape on the wrong side of the veneer yet. I have even stopped putting blue tape on the inside before veneer taping the outside thinking it might leave some residue on the wood that could interfere with the PRG.

Jeff, one would think that if the panels were glued up in the Winter, which some of them have been, when my shop humidity is at 30 - 35% that I would never see seam splits, only puckered seams maybe in the Summer at high RH. I tried a humidifier years and had dust everywhere so that didn't last long. I'd have to go a different route if want to consider that again. Are you using a vacuum bag with yellow glue? If so, how long are you leaving it in the bag, and how long do you let it "cure" before further processing? With PRG, the water added to mix the glue is consumed in the formation of the UF glue so when it comes out of the bag it's dry and rigid, though not fully cured for 7 days. I tried Better Bond once and had immediate seam lift. I probably used too much glue but I never tried it again, especially after I talked with Joe Woodworker and he told me it's not recommended with shop sawn veneer. Now his website says that, too, though it did not when I used it. But I've never tried plain TB or TB II on large panels, just a few small things like a drawer front that I mechanically clamped. I've always thought you need a rigid bond to keep the veneer bonded to the substrate as one unit so they move as one; maybe it's time to rethink that assumption.

Jamie, again, one reference I cited in my first post was a panel glued with epoxy. Also, my friend had a panel of Sapele shop sawn veneer glued with West Systems epoxy on particle board develop a huge split in it - during the first Winter. That panel was made in the Summer in an unconditioned shop, so the RH swing would have been even greater than mine. But I have thought about going to epoxy just to get more open time. Despite what the literature says for PRG, I find I only have about 10 minutes to get it into the bag after putting it on the substrate.

Anyone using polyurethane glue?

John

[Mel Fulks]

John, .....give it the whole TEN minutes ! It's not like 5 minute epoxy, the time does improve it. As I said it was part of
instructions for many years. One of these days I'm gonna call Weldwood and ask why they took it off the label .

[Joe Hendershott]

Wanted to ask you John, and others as well why you do not use cold press veneer glue? While I have done only a few jobs and use commercial veneer have had good results with the titebond brand.

[Kevin Jenness]

Joe, Titebond cold press is a PVA with crushed walnut shells added to prevent bleedthrough when pressing thin sliced commercial veneer. The additive is unnecessary with thick veneer, and the greater movement potential indicates a more rigid glue than PVA.

I generally use epoxy with thick veneer for a rigid glueline and long assembly time. One advantage is that epoxy will be sucked up into the joints between leaves in a vacuum press, bonding them together.

A point that is sometimes overlooked is that epoxy bonds best to rough surfaces, sawn or sanded at 80#, especially when gluing dense woods.

[David Zaret]

i do a lot of veneering with both shop sawn and commercial - what i've found is that i have to keep the shop sawn no thicker than 0.050" or it'll act like solid wood. for glue, the best success i've had is with unibond (2-part urea resin), thickened with the blocking agent. the thickener doesn't impact adhesion, but yields a better, heavier glue layer. with those variables, i've not had issues - the few times i got lazy and used a thicker veneer, i had (minor) seasonal cracks.

good luck!

-- dz

[Robert LaPlaca]

John, I generally don’t use too much veneer for the furniture I build, but I have had really good success using Unibond 800 for the veneer glue.

One project I had, needed crotch Walnut veneer shop sawn veneer (1/16”) applied to drawer fronts and two fairly good sized raised panels, the veneer was applied over quarter sawn Walnut core. Everything looks as good as the day it came out of the bag.

[John TenEyck]

Thanks everyone for the added input.

I have used Unifbond 800. It worked fine but the only piece I have with it is with paper backed veneer so I have no direct comparison to PRG with shop sawn veneer. I moved to Weldwood PRG because I could buy it locally, although now I order it online. Both are UF glues, just in different forms to start with. I don't know if there is any real difference in how well they bond or their physical characteristics other than both form rigid bonds. FWIW, I have used PRG on exterior doors to glue 1/4" thick veneer onto both LVL and stave core cores. No problems yet, though those are narrow and mostly without seams, and hoping there won't be. And before anyone says that's crazy and I should have used epoxy go look up the strength loss vs. temperature for epoxy before you do.

The point about epoxy wicking up into the seams could be a really important one. The panel my friend had the split seam with, using epoxy, might be illustrative in that he preglued the seams with TB. One of the reasons I've never been sold on pregluing the seams is because you can't clamp the joint, you can't even create a rub joint, it's just held together by the seam tape. So the bond is inherently weak, and that's where it let loose in his epoxy bonded panel.

It's clear to me anyway that part of the problem I'm having is the veneer is shrinking more (at least at the seam) than the substrate, or at a different rate which makes sense when you think about how moisture moves through the thickness of a panel. I looked at the cabinets in my house that I've made with commercial veneered plywood; not one seam split. Of course that veneer is only 1/40" thick or so and it's cut with a knife so it's "creped" and inherently weak across the grain, which could be an advantage in moving with the substrate. The glue in commercial veneered panels is at least as rigid as what I'm using. I have no idea how they join the seams however.

I did some more tear downs last evening on panels I glued up a couple of weeks ago. I'll post pictures later today.

Going forward, I'm going to order a humidifier today to try to raise the humidity in my house to something over 40%. 45% would be a good target I think. I'll do the same in my shop with a humidifier that I already have though it may not be big enough for the space. I will let the PRG sit for 10 minutes, then restir, before using it. I'm going to look into moving to West epoxy; it has advantages and disadvantages, like everything. And if I have to replace all my kitchen door/drawer fronts (I hope not but it's not looking good) I will use a different substrate, most likely MDF but maybe BB plywood.

Thanks again. More to follow.

John