Troubleshooting Adhesion Failure in Conversion Varnish Over Lacquer

June 22, 2010

Troubleshooting Adhesion Failure in Conversion Varnish Over Lacquer

Conversion varnish needs a mechanical bond. Here, pros analyze a case where it’s not sticking. May 17, 2005Question
I have been doing wood finishing for a couple of years. I recently stained a maple staircase with Valspar lacquer stain in a dark cherry with several coats of stain. I let it dry overnight, then applied three coats of Valspar conversion varnish. The homeowner called me a week later and told me the finish was coming off. I went to the jobsite to take a look. 99% of the finish was bonding fine. The treads, risers, skirting, and newel post were fine. But on the handrail along the grooves, the finish, when scratched with a fingernail, was scratching down to bare wood. You really had to dig at it, though. What could I have done wrong? What are the guidelines for inspection of proper adhesion of a coating? I’ve been reading a lot regarding staining maple. I cannot figure out how to do a maple staircase with full treads, risers, skirting, balusters, and rail. I do just fine with oak.

Forum Responses
(Finishing Forum)
From contributor D:
Did you sand before applying your CV? If so, with what grit paper? CV requires a mechanical bond to adhere. It’s possible that the open grain of the oak allowed for enough grip, but the closed grain of the maple did not allow the same grip and the failure was in the application process and not the finish itself.

From the original questioner:
Yes, I used 220 3M non-loading grit paper. The areas affected are all in the same area of the handrail and trim piece above the skirt. I tried to fix the spots once, but scratched off in the same areas. The Valspar rep told me I put too much stain on and the surface plasticized, causing poor adhesion, and that I should have done a sample first. I did. How can I fix the problem, and how do you do maple in a dark color with difficult angles?

From contributor B:
I think that rep is right. Too much stain. No adhesion bonding between stain and CV. The way to correct it, I don’t think you want to hear.

From the original questioner:
What bothers me is I bought the Valspar material at Vista Paints. I told their lacquer expert what I was doing and how much stain I was putting on. He told me as long as I could sand the stain, the surface was okay to apply the CV. Well, I’m still learning, but at a high price.

From contributor D:
Did you apply the CV directly over the lacquer without an isolating coat of vinyl sealer? This may also be an issue. You should not apply a catalyzed finish over a non-catalyzed finish. Be thankful that it’s only happening on the rail and not the rest of the surfaces.You may be faced with having to strip the rails. Seeing as the finish is peeling anyway, may not be too tough a job. I would do a sample of your original schedule and materials, only apply a coat of vinyl sealer between your shading lacquer and CV. After allowing the sample to dry overnight, scratch and see if there is a better result.

From contributor J:
I think that even though you gave plenty of time for the stain to dry, it wasn’t in these recesses, thus building a surface that CV could not hold onto. That is why I cringe every time I do a glazed kitchen or hear of one done without a cat vinyl barrier coat. Another thing that could happen is that the stain is wicking out of wood.

From the original questioner:
Thank you. The Valspar expert did not tell me to use a vinyl sealer between the lacquer stain and CV. I think the issue might be the stain quality, because I did the samples with the lacquer stain and CV topcoat and one with gel stain and CV topcoat. It checked fine, but now after the problem with the staircase, I came back to my shop and scratched hard at the same areas of the samples and sure enough, the lacquer stain came off in the same spots, but the gel stain sample did not. Could you tell me a little more about wicking of the wood? I’ve never heard that before.What is the exact procedure for determining proper coating adhesion? Is there an industry standard? I had a gut feeling about this staircase. I have only used CV three other times and it was all oak. I had my concerns about lacquer stain. I should have stayed away. But how do you learn if you don’t try?

I have a small residential company in San Diego. I’ve been painting for 20 years and have been doing wood finishing for two years. I’m not going to give up on wood finishing.

From contributor D:
I just re-read your posts. You mentioned speaking to the lacquer expert. I’m assuming this was someone in the store you purchased the products from. We have a lot of those experts (HA!) here also. I have found that people selling the products, claiming to be experts, are not. You need to use the products in real world scenarios, be confronted with situations and solve problems with chemicals with limitations, and get it right 99.9% of the time. That makes an expert, not reading a label or MSDS sheet or sitting in on a sales seminar.

From contributor M:
As for your question about coloring maple, I use dyes.And a bit of advice: Get hooked up with a company with good tech support. Don’t be afraid to question their reasoning. It doesn’t take much to know more than some of these reps. They may be capable of fielding homeowner questions, but don’t cut it when you have a technical question. In my experience, many of these guys have some experience in finishing and base their answers on their experience, not verifiable, technical information. I asked a rep about using shellac over one of their products. He said, “Yeah, sure, no problem.” Then I said that I would be spraying several pieces and I did not want to be responsible for compatibility issues. Then he got serious and started making some calls. This “expert rep” has sold paint for 5-6 years and then watched someone spray wood finishes. In short, check out the background of your rep/tech.

I wouldn’t have put CV over coats of lacquer, even with a vinyl sealer. Hard over soft, in my opinion, is not a good combination. I would have either shot the toner with clear, being careful not to exceed mil thickness; or shot dye, vinyl sealer, CV, or tinted some CV.

Also, I have had mixed results with Valspar CV. In my opinion, their viscosity is not consistent. While they do have some good products, their tech support stinks. Try calling corporate. It may take calls to 5 people to find out where the product is made, much less answer questions about it.

From the original questioner:
I’m now replacing the handrail, newels, and small trim above the skirt, and I’m considering replacing the whole thing. I can tell the owner feels the whole staircase is failing. It’s not, but it’s hard to argue with him while I’m standing there scraping finish off his handrail with my fingernail. If I keep going with this staircase, I’m losing money there and now I’m losing money on other jobs. I should just buy my way out. I did learn not to spray CV over lacquer stain. Won’t do that one again. I would like to try a different method but I don’t even know what dye stain is. I do know that most stains have pigment and dye. Where do I get dye stain, and how do I do the maple with confidence? The way I’ve been doing them, even when it was just regular nitro lacquer and lacquer stain, the maple staircases are just too difficult. I would be on my last coat of clear and bump the rail going all the way back to white maple. I thought I was going to burst a blood vessel. I think I will stay away from the maple, for now. Oak, cherry, mahogany, no problems. And I’m doing my first alder cabinets next month.Maybe I’m using the wrong product to produce my color? Regardless of the binder to adhere it. I’m not very good with UTC tints. So I have two choices: oil stain or lacquer stain. As far as adding the color to the clear, I don’t understand

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