Troubleshooting Finish Cracking over Crotch Mahogany
Unpredictable wood movement in crotch veneers can lead to problems with finish performance. May 17, 2010
Question
I completed a kitchen with crotch mahogany center panels a year ago, and the panels have developed fissures and what looks like white hairline cracks. We applied the stain, barrier coat, three polyester basecoats and two acrylic topcoats as recommended by the distributor. Is there anyone else who has had a polyester problem? More importantly I need to sand and refinish, but am unsure how to achieve a filled grain finish over crotch mahogany without failure.
Forum Responses
(Finishing Forum)
From contributor R:
Do you know how the veneer was glued? Was it hot pressed, vacuum bag or contact cement? We should be able to rule out dry film thickness, so what is the environment where this kitchen is? Does it get very cold or have a lot of temperature and humidity fluctuations?
From contributor N:
Did you use the same schedule on all sides of the project? I have seen tables done with polyester sealer and topcoat on the top side only cup and crack.
From contributor B:
1. Did you use a wiping stain?
2. Did you let the wiping stain dry overnight?
3. If stained, how long did you wait before applying the Isolante?
4. Did you spray the isolante and let dry overnight or did you spray a wet on wet polyester sealer over the isolante?
5. If you sprayed wet on wet sealer over it, how long did you let the isolante flash off before spraying the sealer?
6. If you sprayed wet on wet sealer over isolante, what solvent did your distributor tell you to use?
7. If you let the isolante dry overnight, did you scuff it before applying the polyester sealer?
8. If you scuffed the isolante, what grit did you use?
9. If you scuffed the isolante before applying the sealer, did you blow the dust out of the grain before applying the polyester sealer?
From the original questioner:
I used a cold press PVA glue and pressed them up in my vacuum press. The climate in the condo is very controlled with Aprilaire as well. I did not use the polyester basecoat on the backs of the doors. Just a sealer and acrylic topcoats. The front panel was the crotch mahogany and the backside was rift cut mahogany. I was told the panel would still be in balance.
From contributor B:
I don't believe there is any correlation between using different sealers on the backs and the fronts of these doors. If the issue was warping, this most certainly would be the culprit. To be honest, I am surprised that it wasn't an issue. The climate inside this condo must be very finish friendly.
From contributor C:
I have found that unless my substrate is the same species (in this case mahogany) or poplar, there are issues with movement and cracking. All crotches/burls/etc. with wild grains seem to have this problem to one degree or another - multiple stresses running in varied directions. If you laid this up on another substrate such as maple, birch, beech or other close grained wood, you'd be better off remaking them with a substrate of the mahogany or poplar.
Other questions come to mind about the type of stain/pigment/dye/blend. Is the stain made for going under the polyester and sealer - is it from the same company as your coatings?
When I finished yachts which were in a much more severe environment (south Florida), I learned the hard way to use polyester pigments and dies that could be catalyzed after being added to the clear coats or mixed with the manufacturer's stated solvent to know for sure I had total compatibility.
I prefer to use a 2k urethane designed for oily and problem woods called Rosewood Barrier Coat from Seagraves. I can't say it's the ultimate because I have not used all isolation coatings out there, but in over 20 years of using it I never had a blowback. It is a slow dry sealer with excellent penetration and build, much more so than other isolante types I have used.
You have a problem for sure, but at least you're not having to redo a yacht interior all over again.
From contributor J:
I don't know a heck of a lot about finishes, but I do know a lot about veneer, so let me throw this into the mix.
Crotch wood, especially mahoganies/khayas, are like living beings. They continue to move for an unbelievable amount of time, even in veneer form. Early in my career I made two mahogany crotch veneer panels for my own kitchen using yellow glue, so I get to watch their progress every morning while I drink my coffee. Still moving.
Look at your panels closely. Is it possible that the veneer has moved/checked and cracked the polyester (which is pretty brittle as finishes go), leading to the white lines?
I would not consider laying up crotch veneer with anything less rigid than urea formaldehyde glue, and we always crossband with longwood before applying to the substrate. Those who know even more than I have advised squeegeeing marine epoxy onto/into the veneer after layup and before finishing to help stabilize the veneer. I pass that advice on to our customers, though I have never tried it myself. We laid up about a hundred crotch mahogany door panels for a customer last fall and he recently told me that two of them were showing some checking (he didn't use the epoxy treatment, for what it's worth). The two narrowest panels and all material was from the same flitch! Wood - even in veneer - can be wild stuff, and crotches are some of the wildest.
From the original questioner:
The staining process took two days, was wiped on, and was allowed to dry overnight.
The barrier coat (isolante) was allowed to dry overnight and then sanded with 150 grit sanding sponges which are really about a 320 grit. My finisher is not sure but I think the barrier may have sat overnight after sanding. A problem, I'm assuming. The dust was blown off between coats.
From contributor C:
I'm in full agreement with contributor J on this - any trouble I have had is with burls, walnut and mahogany especially. The slow dry rosewood barrier coat I recommended works the same as contributor J's epoxy method. You need to get some down into the structure of the wood kind of like they do with pressurized treatment of knife handles with acrylic, where it stabilizes the entire piece and prevents it from moving as much as possible.
With boat work, I never stained the wood directly - all color was toned on after the rosewood barrier coats and polyester were worked out, then a coat of 2k was applied and toned on over that with appropriate dye/solvent/2k mixture. Only enough to color up the panels to match each other's variations.
From the original questioner:
I can't thank all of you enough for the input I've received. It's really helped give me an idea of where to go from here.
From contributor E:
"I would not consider laying up crotch veneer with anything less rigid than urea formaldehyde glue, and we always crossband with longwood before applying to the substrate."
Best finishing advice you'll ever get.
From contributor C:
Resorcinol is just as good, but you're correct it takes a rigid glue for worry free results. All PVAs move; epoxies, a small amount. For yacht work/car dashboards/other extreme environments, resorcinol and UF glues definitely! Ditto on the crossbanding.
From contributor M:
"We always crossband with longwood before applying to the substrate". Can someone explain what that means?
From contributor J:
This means that the crotch veneer is pressed onto a non-crotch veneer (longwood = wood that looks like typical lumber) before being pressed on to the core, or substrate. This helps strengthen and stabilize the crotch veneer because wood has high longitudinal tensile strength (it doesn't want to stretch in the direction of the grain).
