I seem to be in a pickle again. A few days ago I laid up 2 sheets of 3/8" MDF with birdseye maple (face) and flat sawn maple (back), using West epoxy as my adhesive and a vac bag as my clamp. The veneer is paper backed. Today I started cutting the sheets into cab door panels and discovered that I could pull the bird veneer off the MDF, in a single piece, without much effort. The flatsawn veneer was more tenacious, but I was still able to pull much of it off. There is no lifting at the edges of the veneer nor are there any bubbles in the panel. Except for being able to pull it off, everything looks fine. I double coated the MDF with the epoxy mix and single coated the veneer paper. I used a foam roller for uniform coverage. Shop temp was around 60 and my vac was set at about 10" Hg. (5 psi +/-). I had good vacuum and an overnight cure before removing from the bag. I did this twice over a two day period and the results are the same for both sheets. I've done this many times and never had an issue, but I've never used MDF as my substrate.
I lightly sanded the MDF with 120 and blew it off prior to applying the epoxy. After pulling a piece off a scrap panel I can see what appears to be wood/paper fibers adhered to both the MDF and the veneer paper. A light scraping of either produces a small pile of wood/paper fibers. Any ideas of what went wrong here? Any thoughts would be greatly appreciated.
From contributor N:
Have you used the west system for stuff like this before/I use epoxy from US composites and it does not achieve full hardness for several days.
As far as mixing goes, yes that can be an issue. I had a friend mixing for me and he too has quite a bit of experience so I'm doubtful that that is it. All of the stir sticks, rollers and pots are covered with thoroughly cured material. Nothing even slightly soft, and they were put outside where it was cold (30's) for several nights. I too have some suspicions regarding the paperback veneer, but I've yet to have it delaminate. When using epoxy it really does eliminate bleed-through, which would be nearly impossible to remove entirely.
In my bag I used cauls on both faces of the layup and a breather fabric on top. The layup looks really good, it just doesn't stick very well. Upon really close inspection today, with good light and a magnifying lens, it looks as though the bond between veneer paper and the top few atoms of the MDF are good. There is a very thin but uniform layer of fibers stuck to the veneer paper. It would seem that a couple of mils of the fiberboard just peeled off. Is this possible? My first suspicion was, and maybe still is, a starved joint. I double coated the MDF because the first coat did seem to soak right in, so maybe a second coat still didn't do it. I used over 1/2 gallon of adhesive to lay up two sides of two sheets (128 sq/ft). Seems like it ought to be enough. Both panels, done on two different days, were put in the bag with the flat sawn veneer (the cheap stuff) on the bottom and the bird (the really expensive stuff) on top. It is only the bird (the really expensive stuff) that is coming off.
It seems that the best approach is to rough up the MDF and coat it once with epoxy to essentially saturate the first several millimeters of the MDF, and allow it to dry. Thus inhibiting a further sponging deep into the material. Since MDF is so very absorbent, you should be able to follow standard procedure with a second coat of epoxy, veneer and vacuum bag and achieve a very good result.