I built these two small tables for a designer about six months ago. The mahogany crotch veneer was quite rippled, so I sprayed it with veneer flattening solution and pressed it between paper, repeatedly changing the paper until it was fairly dry, though slightly damper than when I received it. (There is a window for pressing after flattening before it dries out and curls back to original state, two or three days.) I pressed them onto MDF cores in my cold press using UF glue. The designer stored them in his normally heated office space. The cracks and slight blisters you see in the photo resulted. It seems noteworthy that the same area on two separate tables shows the problem, presumably meaning that part of the tree was under extra stress. I would like to get opinions on whether I did something wrong, or whether difficult veneers just occasionally fail in this way. Also whether it’s possible to repair. I’m assuming finish will have to be stripped back as a starter.

View higher quality, full size image (4032 X 3024)

View higher quality, full size image (4032 X 3024)

View higher quality, full size image (4032 X 3024)

I feel for you. Figured veneer can be so unforgiving. I haven't had a similar failure but have had to fill and repair small splits in crotch walnut. It will be up to you and your client whether you can make repairs to your mutual satisfaction. I suggest that you repair one representative area with fillers and tinted lacquer and make an assessment of the time and quality of the finished product.
The blistering should be easily repairable. I made a plastic platen that I hook up to a small vacuum pump to press small areas. It is essentially a 1/2" piece of polyethylene sheet drilled and tapped for a hose fitting and a neoprene gasket in a groove. Perfect for spot repairs.
What caused this? I don't like pressing on mdf. Many do but I fear that the mdf fails under the veneer. I have pressed moistened veneers with success so you should have been okay with the high humidity softened veneer. Did you have any bleed through of glue? It is possible that you did not have enough glue. The veneer has sections of essentially end grain that has a weak bond with the substrate unless the fibres are fully saturated. This has the added benefit of stabilizing the veneer. The areas of blistering indicates insufficient glue. Normally I don't like any bleed through but with figured weak veneer it is desirable.
I hope this helps. Good luck

Thanks for your reply. When you have reglued small areas, the fresh glue has always bonded with the dried glue? Are you using UF glue? I was thinking heat might help since UF is thermosetting, so was planning on injecting glue and then using an iron. I could also clamp the area. Do you use Xacto blades or a syringe?

Re MDF as a substrate: what do you prefer over MDF? Plywood doesn't seem suitable because of the many bumps and unevenness, and I don't have a widebelt sander to calibrate the cores. Do you use particle board? One thing about MDF is it does drink up a lot of glue, so you may be right about insufficient glue on my tables.

I used to work in a shop that hot pressed UF glue. I reacted to the fumes. I don't use it now. For you I think you should continue to use the UF for the repairs. I use a syringe and thin feeler gauges when I need to work glue in under veneer. I will slice a slit through the bubbled veneer, lift it with a pocket knife and work in the glue. Massage the glue around and then a warm iron will set it.
I prefer particle core. It is what we used in the shop that I learned veneer work and I continue to have good results with it. I use water resistant pva hot and cold pressing.
Proper glue amount is a bit tricky sometimes. The substrate, veneer type and shop temperature can have and influence so it is best to err on too much glue or even better do a test pressing. This could be finish samples so use the same substrate and glue in your samples and same press method if possible.

Checking of crotch figure is a problem that goes back a few centuries. Using a wood veneer backer (2 ply veneer) is the best insurance. Highboys from the 1700s in Massachusetts had the same problem.

Your f'ed. When you sand you are going to burn thru to the substrate.

I agree with Rich - Old problem and 2-ply is a good strategy.

I would amp it up a bit and use epoxy (West System) and two -ply it to Af. Mahogany or similar.

By golly, that'll do it.

To be clear, testing samples would not have helped because the bond seemed fine during the time they were in my shop and afterwards during finishing and storage. My usual practice is to wait until the glue is cured, then thoroughly wet down the surface when removing the tape, checking for any bubbles. I’m going to try to repair and have the finish stripped and resprayed. Any opinions whether to strip before or after ironing down the bubbles?

Makes me nervous when you say "fairly dry", "damper than when I received it", and "thoroughly wet down the surface when removing the tape". You talk a lot about added water. I never glue it done unless it's bone dry. I just keep it tight in the cauls and paper until it's really dry. I use a slightly damp small sponge to just dampen the tape and then use a card scraper. If I even check for bubbles, I just use a strong indirect light. I'm guessing that veneer did a lot of changing from all the water you appear to put on it. Not surprised you got bubbles now.

You could be right about the moisture, that is why I highlighted the procedure for veneer flattening. My bible is The Complete Manual of Wood Veneering by William Lincoln. It’s his recommendation to wait 24 hours or so after pressing to remove the tape and apply water over the surface to look for bubbles, something I have done for years without a problem. As for the flattening solution, all sources I have read said there is a window of about two days from treatment to pressing. I spray the solution with a pump bottle, but some sources even mention dipping veneers, which would end up with more moisture. Typically I have to change out the paper interleaf two or three times, with pressure in between, until the veneers seem subjectively dry enough. I have the Delmhorst two pin moisture meter, but not sure if that could work on veneer. It seems there may be a very fine line between dry enough for pressing and not reverting to the difficult buckled state. Thoughts anyone?

I've never found that a short window was necessary after flattening. I also prefer not to make a subjectively dry enough rating on my work. How much would a pinless meter cost compared to redoing the layup on that work? Buy a new meter. This was the last piece I did with mahogany crotch figure.

View higher quality, full size image (2144 X 2972)

Nice work. Thanks for the suggestion for the pinless meter, I wasn't aware that was what to use for veneer. Is there a particular one you favor? Are you checking moisture after flattening, and getting to 6% before pressing?
As for a crossband, I haven't found much info about that. I assume it should be the same species but unfigured, laid up at right angles to the face. Is there any particular order recommended, i.e. pressing crossband plus face, front and back in one pressing, versus making up the two-ply veneer first, then using a second pressing to glue both to the core?

Splice up your faces and press to the cross band. You can even do preliminary sanding at this point. Next press front and back to your substrate.

I will ask: Should the two-ply be the same grain direction as the crotch? Or should it be 90 degrees to the crotch, as in true cross band?

Yes, I know the grain goes every direction in a crotch, but the crotch exists with a generalized grain direction as in the tree from which it was taken.

My question about crossband direction too!
Also, trying to understand the rationale for two separate press runs for two ply, which adds to my shop time, since I have a cold press. Isn’t there an advantage to presenting that crossband ply with glue on both sides at the same time, effectively equalizing the moisture content for that ply?
Also, any opinions on polyurethane glue for pressing crotch or burl veneers? Not sure about the bleedthrough issue, but it has no water to create moisture havoc, and is easier to use than epoxy.

I always run the backer veneer 90 degrees to the face veneer. The only time I didn't was when bending the veneer over a large radius. I ran the backer at a 30 degree angle so it had enough flexibility. The absolutely only time I use polyurethane glue is when gluing metal to wood. That's it. When that glue first came out, I glued up an exterior door. Huge golf ball sized globs grew from the stile and rail joints and I worried for years about a panel cracking since it should have definitely glued in the panel. I only glue the two layers of veneer at the same time, never on the substrate at the same time. Just tradition I guess, but I would worry about everything shifting around when trying to lay up 2 layers on top and a layer on bottom. If you make the whole thing oversized enough, I guess it would work.

I should mention, if you want to save labor hours, contact Indiana Architectural Plywood and get a quote for the 2 ply layup.

I agree with Rich also, we always did a 2 ply cross band on any crotch or burl veneer.