Discoloration of Conversion Varnish

Acidic conversion varnish formulas can react with metals in the application system or with wood chemistry, causing a red, black, or gray discoloration. Here are some suggested precautions for prevention. March 29, 2006

We have had a couple of jobs turn fire engine red on oak and cherry. It seems as if it is product related. Has anybody had this problem with conversion varnish? We have tried to recreate the situation with contaminants but no luck.

Forum Responses
(Finishing Forum)
From contributor A:
I have seen this with iron contamination of the acid cured coating. The most likely source is the can, bucket or plumbing and pick-up tubes on the spray equipment. The acid leaches the iron from any mild steel components in the system and applies the iron to the surface along with the coating. Over time the iron rusts within the cured coating. Usually this occurs when the components have been in use and broken down considerably or where the acid is very strong or the catalyzed material is stored for long periods in contact with the mild steel.

Prevention is achieved using stainless steel components approved for exposure to acid as well as resistant plastics in all wetted applications. No empty mild steel buckets, pails or cans should be used to store, measure or dispense acid catalyzed materials once activated. Even those pails with a thin phenolic paint liner are not acceptable once the coating is catalyzed. Also, avoid aluminum as it pits and dissolves in some acids. This condition is not dependant on wood species. The solution to your problem on the installed product is to redo. It is not a reversible situation.

From contributor B:
I've seen CV develop white lines in the finish over time. I have no idea what causes it. It's almost like scratches but they aren't. I solved the problem by going back to a precat. Finishes are great and no customer complaints.

From contributor C:
Some timbers will react with the acid catalyst. Pine heartwood will go bright pink or deep red. Cherry is also a problem. I've never seen the problem on European Oak or American White Oak. If iron was the problem, the stain would be black or dark grey, especially on the Oak, due to the formation of iron tannate. The way to prevent it is to use a non-catalyzed sealer. We use one for our floor varnish which is based on PVB (polyvinyl butyral).

From contributor D:
It may be the metal of the container reacting with the acid. Some of the can makers are using lower quality materials and the so called lined cans are not holding up like they used to. We have instructed people to put varnish into stainless containers or a five gallon can from an older batch of varnish, since it seems to hold up just fine. Some have even used plastic but I don't know how that would affect the varnish, if at all.