Troubleshooting Flaking Conversion Varnish

Likely causes for a flaking finish: sanding techniques, or too heavy a primer coat. March 16, 2015

Question (WOODWEB Member) :
We are having an issue with MLC Resistant pigmented CV flaking off after six months-one year. It easily flakes off as if there is no bond to the wood at all. We sand with 150#, prime, then scuff with 320# and apply one coat of Resistant. We achieve a total dry mil of 4-5, including the primer. The finish is extremely brittle after the mentioned time and releases from some edges then flakes away. Does anyone have any ideas about what’s going on?

Forum Responses
(Finishing Forum)
From contributor K:
Have you tried scuffing with something a little more coarse to give the resistant something to bite into? Try 220. Also, what primer are you using? Make sure your using the primer that is intended for resistant (Clawlock).

From contributor K:
After re-reading your post a couple other things come to mind. Four-five mills in two coats is an awful lot of dft. If you are pouring on your primer that could be the problem right there. In your post it says the paint chips off like there is no bond to the wood, not to the primer. So when the paint chips off you’re seeing wood? I would suspect somebody is laying on the primer too heavy.

Try this (it takes a little longer but your primer will bite). Dilute your primer a little more than usual. Make a quick pass with the diluted primer. You want to see all the grain still with this pass. This will allow the primer to drop down into the pores and really grip. Let it flash off then go back with a heavier coat, but not too heavy - a nice coat to achieve even coverage. Sand it back with 220 then give it another nice even coat of primer. You should be able to get the surface perfectly flat without burning through the primer at this point. Then scuff with 220 and apply your resistant. The problem with spraying too much primer too fast trying to get things done in two passes is that the primer actually bridges over the grains instead of seeping down into them. As the solvents evaporate, the primer shrinks, pulls out of the pores and dislodges from the wood.

From contributor X:
You say it's flaking off the edges to bare wood. I would consider what's going on with the edge itself. Are those profiled edges that got machined with a shaper/molder/router? If so, was the tooling dull? Maybe the edges were burnished and became too slick for the finish to bond to. Dull tooling and closed grain wood like maple can cause adhesion problems.

From contributor M:
I've sprayed a lot of Stealth and Resistant (our main product for several years) and have always used 180-grit on an orbital. I've never had this particular problem. My number one suspicion in your case would be that your guys are using the sanding pads way beyond their useful life. I probably discard twice as many sanding pads as the average finish guy, but I never have adhesion problems, and I use 180 almost exclusively. If your guys are using the pads once they get worn out, and if they are using a lot of pressure (making lots of heat) then the sanding pads are just burnishing the wood.