Timing with putting on backing veneer

[Alan Lightstone]

I've always wondered the answer to this question. Typically I'll veneer both sides of a piece at the same time, put them in a vacuum bag, and voila'.

But the present piece I'm working on has one very large surface which would be a bear to get glued and aligned quickly enough on both sides before putting in the vacuum bag.

Do I risk warping the piece too much if I do one side, put it in the vacuum bag for 4-6 hours, then veneer the other side a day or two later and do the same thing?

I was thinking that any induced bow will be reversed by the moisture of the other side plus it's time in the vacuum bag, but is that true?

How do you guys do this on large pieces (the smaller pieces I'll just do both sides at once. That's not a huge deal.)

[Jamie Buxton]

I don't have an answer to the basic question, but I've worried about it too. One work-around is to use glue that doesn't contain water -- epoxy. It also has a very long open time, so you have plenty of time to get the veneer in place and in the bag. I use West System epoxy which I get at a local marine-supplies place. It is available over the web too. System Three, as far as I can tell, is the same stuff. Woodcraft sells it.

[Thomas S Stockton]

Alan
It will warp and the bigger the piece the more it will warp. Use a slow setting veneer glue, I use veneer bond dry resin from www.pro-glue.com, it has a 15 to 25 minute open time which should be plenty. Weldwood plastic resin glue and unibond also work.
Also read the directions these glues are more sensitive to minimum temperatures than yellow glue.
Tom

[Peter Quinn]

I feel your pain. I can remember the short time I spent working in a veneer shop, giant hot pneumatic press table, 5X10, we were gluing up doors almost 3'X7'X1" thick using brutally expensive macassar ebony, substrate was VC plywood, using a water based PVA glue made for hot press, a little like titebond III, time to cure was 7 minutes in the press. Had to glue both sides at once to keep from warping what would be large inset doors. As soon as the macasser (large sheet of 3-5" QS veneers stitched together) hits the glue.....it goes bonkers like a wavy cut potato chip. Trying to tape it down as it curls into oblivion. Shop owner/guy teaching me/rammy x-n-such ignored my suggestions that we should mist the veneer, or wipe it quick to keep it flat as it hits the glue. So you wrestle the first layer on, flip the big sheet over...clock ticking....glue the other side at 150MPH with a roller/hopper....paper on the outside of both faces to catch any bleed through so it doesn't get stuck to the platens and ruin the very expensive machine.....now you are wrestling a large paper-veneer-1" core-veneer-paper sandwich intent on wrinkling like an accordion into a 150 degree hot press. Glues open time was theoretically 9 minutes? I'm asking "have you ever tried unibond? I didn't last long there, or couldn't stand any more fun and left, but I did learn how NOT to do a few things.

Its always best to be realistic and use a glue whose open time matches your square footage. And at a certain point it really takes two people to wrestle a large glue up. I think I could probably get a full 4X8 sheet veneered and vacuumed with 2-3 guys in under the 11 minute open time of titebond III or a cold press glue, but that would be some of the longest minutes of my week. So consider the plastic resin glue, and if the veneer is thin and the surface area large, consider a glue with no added moisture like unibond, or develop a method to mist the outer face of each veneer just before laying it in the glue (which pretty much takes two people on a large sheet too.) When you mist it it will start to curl, when it hits the glue it will flatten right out because now both sides are wet. Or you can lay the veneer down on the glued substrate and moisten the veneer as needed. Be very organized, have the bag propped up with some holders from the ceiling, use a good glue roller sized for the work. Go very fast! It can be done. But sometimes that second person is just not available, let alone the third.

To answer the original question, I never veneered one side of a piece of flat work because where I worked I was told not to, and I have yet to encounter a piece large enough in the home shop or other jobs to require it. But.....I'm just finishing up a pile of curved in plan entry doors that are made with marine grade veneer cores that are solid edge banded (1") then skinned (.150") with mahogany. So I have an in bag bending form. I bend the cores on the form with a filler to match the missing skin on the concave side. Then edge band and flush trim. Next comes veneer...but you CANT veneer both sides at once, doesn't work. The slightest deviations in the form cause the concave (down side in this case) face to not become glued, doesn't mate the form perfectly, no pressure no way with the rigid veneer cores. I have to veneer the convex face...leave in bag 2hrs-2.5hrs....beyond that is really not necessary....then pull the forms, flip the cores now veneered on one side, veneer the other side this time using a piece of window screen as a breather mesh/top platen. I leave the second glue up an additional 2.5 hours. Bag pulls down hard on the concave face...perfect glue bond. And initially this worried me, because I was violating the veneer both faces at once rule. Glue was titebond III. But no failures, no warping, no odd spring back, all parts match their intended radii within acceptable tolerances.

What to learn from this? Moisten one side of a thin veneer....it goes crazy. Moisten both sides.....fairly little effect. Moisten one side of a piece of plywood....you get a pringles. Throw a sheet on the lawn some morning to test this. Moisten both sides....minimal effect, still in balance. Moisten one side of a core, veneer it, put it in a press....pull it out, veneer the other side quickly, put it back in press, pull it down hard....may just work. I was never told if the veneer both faces at once rule was more expeditious (it was a business after all, at least in theory), or a strict requirement for success, or perhaps the important variable was the hot press? I'd be tempted to say try it on a small scale, take some cheap backer veneer and a 2X2 scrap of core material similar to what you will use, and run through your method.

I wil say I was taught to let the veneered panels cure vertical, not horizontal relative to the main face, and that seems to make a world of difference. Lots of guys pull them out of the bag, lay them on stickers on the bench flat.....bad idea. The vacuum doesn't let the moisture in the glue leave. When you pull out the panel and lay it flat the moisture in the up face wants to leave quicker than the down face....not great. But if you stand them on edge using a wood screw clamp so both faces are exposed for a day or two, they stay very flat....a little air movement in the way of a small fan seems to help expedite the process.

[Kevin Jenness]

If you veneer the other side before the moisture in the first dissipates, you will be alright. With Titebond 1 at 70 F you can pull the panel after 45 minutes or so, flip it and glue the other side and put it back in the press.If working alone, you may need to quickly trim the veneer overhang in order to flip the panel safely. Better to do it in one shot if you can get help, but not out of the question.

As stated, a slower setting glue will allow a single pressing on a large panel. An electric blanket can speed up the cure of epoxy or UF glues if speed is an issue. UF glues typically need a minimum of 70 F to cure properly. Beware of epoxy bleedthrough on raw veneer (UF glues can bleed through too). Test panels are always wise when using a new adhesive.

I would be hesitant to add water to the veneer face to prevent curling- the less water in the panel the better. If curling is an issue, work faster or use a different glue. Generally, if you lay the first glued face down on the veneer its weight will hold the veneer flat, and the caul will do the same for the second face until it is under pressure. I will say working with a frame press decreases the stress of "getting it in the bag" in good order.

[Alan Lightstone]

Ugh!!!!! Not the answers I was hoping for (but great information, guys).

This is a 65 x 28 sheet of 3/4" plywood with expensive (and pitifully fragile) teak veneer.

Working myself, so not sure how I'd be able to flip it. Every time I look cross at the teak veneer (or handle it, really) I get cracks I have to tape.

Was planning on using better bond veneer glue, so it has plenty of water in it.

Unfortunately, my Unibond solidified, so I can't just use that, and was hoping to get it done this weekend.

I do have Better Bond Ultra-Cat PPR glue, but I hate mixing the stuff up, and it, at least in my hands, is far more difficult to use than the normal Better Bond Veneer Glue.

Anyone know any good sources for Unibond 800? Or hints for using West Systems Epoxy (I have tons of that at home.)

Or for that matter, will Unibond 800 solve the issue of doing one at a time, or is the water content in it enough to potato chip things?

I've never veneered with epoxy before (though I can see it being a good choice with an oily wood like teak).

[Max Neu]

Just curious, what is this piece being used for? If it is a table top or something else that is getting fastened down, I wouldn't stress out about, you can easily draw it down flat. If it's too much to do in short order, press one side late evening, then get up early and do the other side. I am assuming something that big is getting fastened down to something to straighten it.

[Alan Lightstone]

It's for the back of a desk. It will be fastened down at the sides with the side pieces, and the top, but the middle will have a wide stretch that isn't fastened down, and has doors, so warping will be plenty noticeable.

[Kevin Jenness]

1. epoxy:Longest working time. No water. Will undoubtedly wick up through thin veneer in places (grain runout, curl, pin knots) in a vacuum press. May be unobjectionable after thorough sanding. Do a test panel. Can be used as a filler, squeegeed onto face and sanded back post-layup- this is a pain, but can give a consistent appearance and can be useful with void-filled veneers like burls. Needs abraded surfaces (80#) to ensure good bonding.

2. Unibond 800: Not entirely water-free. Mayl be less prone to warping the panel if laid up in two stages than pva. Available from Vacuum Pressing Systems, which has a very informative veneering forum. Be sure to follow mixing instructions carefully. Do a test panel to assess bleed through; adjust glue spread or add blocker as necessary.

3.:PPR glue: More water content than 1 or 2. Reliable and decent working time. More rigid than pvas like Better Bond cold press glue. Easily mixed with a drill paddle sourced at the hardware store. Do a test panel as above.

4.:Flipping over: You are going to be flipping the panel at some point, the question is whether it has wet glue and/or veneer on it. Whatever adhesive you use, if you can get the layup in the bag within its working time, doing both sides in one session is going to be the most efficient and have the best chance of coming out flat. I recommend laying the back veneer on your cookie sheet face down, rolling glue on your substrate, then flipping the panel onto the veneer. 1/3 sheet of 3/4" ply is not that heavy. You may want to fasten some positioning blocks to your veneer/ cookie sheet to help get it where you want it. A block with pins projecting for a handle may help keep your fingers out of the glueline. Once flipped, coat the top face, lay on your face veneer and caul and apply pressure. Do a dry run to make sure you are comfortable with the process. Once the first face is laid on the veneer you will have a very difficult time shifting it. You may want to run some strips of tape across your veneer faces to reinforce them for handling.

Given the glues you have in hand, the PPR will give you enough time for this. If I were set on using a pva, I would use Titebond 1 Extend for its slightly longer working time and more rigid glueline, and I would be able to do both sides at once with a panel this size. If I felt I couldn't work fast enough, or wasn't comfortable with handling a glue-coated panel,I would do as described in my first post- pull the panel as soon as it has tacked up well, trim the overhang, flip over and do the other side. You can assess the curing time with a small sample clamped lightly within a plastic bag to simulate the conditions in the vacuum bag.

[Alan Lightstone]

Well, I tackled the project last night. I tried a test run to do both sides, and wound up ripping the veneer in multiple locations. Too fragile. So I bit the bullet and did it in two stages.

Also, I was concerned about my PPR glue being past its shelf life, so I chose not to use it. That left me only with the Better Bond cold press glue, which I used.

I veneered the one side late last night, then put it in the vacuum bag. I then went to sleep and it stayed there for 8 hours (way longer than needed / perhaps desirable, but needed some sleep.)

This morning, I took it out of the vacuum bag, trimmed the excess, and flipped it over, cut the new veneer pieces, taped them, glued the second side, and put it back in the vacuum bag.

All-in-all, I figure about 2 hours out of the vacuum bag before it went back in for the second side.

The good news - the panel looks very straight. No warping that I could see, either after 1 side was veneered, or after both sides. So I either caught a break there, or my process avoids that issue.

I think I'll order some more Unibond 800 tomorrow. I wasn't fond of the fact that it went bad so rapidly last time I bought some, and has such a short shelf life, but I remember it was great to work with and made a great bond.

Regarding the epoxy, the veneer was so thin (I swear it gets thinner and thinner every time I order some), I would be concerned that any epoxy bleed through would risk sanding through the veneer to eliminate.

Anyway, this was the toughest / largest piece. The others should be no problem to veneer both sides at once.

[Kevin Jenness]

The shelf life of both PPR and Unibond 800 can be extended considerably by keeping them refrigerated.

[Chris Padilla]

I use BB glue all the time. I was going to suggest you do exactly what you did: either leave it in the press nice 'n flat or take it out after 45-60 minutes like the instructions say to and then throw a buncha weight on top and leave it until you get to side 2.

Yeah, the shelf life of Unibond is irksome. I once ordered some for a project and was delayed with the project. The 800 was toast before I could get to it. A full unopened jug wasted.