Troubleshooting Cracks in a Conversion Varnish Finish

Cracking in a CV finish might relate to excessive thickness. June 9, 2012

I work for a small furniture company and finished a center table for a client approximately two years ago. The top of the table is MDF, with a 1/8" veneer (don't recall the species - something inexpensive for paint-grade pieces like poplar). The finish starts with a couple coats of ML Campbell's Clawlock (white primer), then gets some glazes, then top coated with a couple coats of ML Campbell's Krystal conversion varnish. This combo has always worked great for us before.

We had an issue about a year ago with a seam in the top showing through, so we brought the table back, filled the seam, then instead of stripping the top, we sanded it flat, sprayed more Clawlock on top of the existing Clawlock, then continued the finish like normal, ending in a couple coats of Krystal. Problem solved...

Except now the client has sent the table back again. The seam still looks good and flat, but now there are several inch-long cracks in the top all running parallel to the grain in the veneer. I tend to think that maybe Clawlock and Krystal aren't as flexible as say a pre-cat, so when the wood expands and contracts, it has no choice but to split instead of moving with it? I hope this isn't the case or we will have many returns in the future.

I have wondered if perhaps we used too much Clawlock once we added more for the repair on top of what was already there. Any thoughts? After being returned for a second time, I have to know that the product I am giving them will last.

Forum Responses
(Finishing Forum)
From contributor R:
Usually cracks that run with the wood grain are caused by movement in the wood. Cracks that run across the grain are caused by too much coating weight. In your case, I would say the wood is moving too much for any pre-cat or post-cat system. They both dry hard for durability, but will pass cold check tests for movement up to a certain point. Sounds like a substrate issue.

From contributor J:
Why not lose the veneer and spray right on top of the MDF? As mentioned, it sounds like veneer checking.

From contributor M:
Sounds like you ran right into the "mil thickness intolerant of CV." A few years back we refinished an 85" conference table that the finish was cracking on. The table was finished at the factory and delivered and assembled (4 coats). The customer did not want to see the joining seams, so the maker sent their finish crew to re-seal the table (3 coats). The customer did not like some dust in the finish, so they sprayed on a few more coats. The finish cracked along the grain of the veneer in six months. We stripped it and applied 2 coats of ICA vinyl and 3 coats of ICA cat acrylic. Problem solved. Some CVs are less prone and some are more. Keep it to 3 mils or less. Or pray when you need to apply more than 2 coats of CV.