Sanding for Adhesion with Conversion Varnish

      Catalyzed conversion varnishes require a mechanical bond between coats. Here's advice on sanding techniques that help avoid burning through coats you've applied, but still achieve roughness required for a good bond. June 28, 2013

The brand I use (Lamar) states to sand. Between every coat, do any of you do that? Can't you recoat as soon as it flashes?

Forum Responses
(Finishing Forum)
From contributor F:
I guess the best way to put it is "only sand the parts that you want the next coat to stick to." Pre-cats and post-cats have to be sanded to provide mechanical adhesion.

From contributor N:
I would like to get some answers on this too. I started using CV this summer and sometimes wonder how fine a grit you can use and still get adhesion. Whenever I stain a project, there is a fine line between sanding through the edges and not sanding at all. I have not had a problem yet but it is very time consuming to sand a profile and not sand through. Maybe I don't know what I'm doing but I feel like a damn surgeon trying to sand the edges. At least lacquer burns into itself and I can wait till I get some build before I sand. Any help would be appreciated.

From contributor U:
Even when you are sanding with a fine grit sponge or paper, you are making the scratches that a catalyzed product requires to hold on to. The pre cats and especially post catalyzed conversion varnishes begin to do their job of chemical and moisture resistance once applied. There is no golden window of time that you can stack coats on without inter-coat sanding. If you rush coat after coat, trying to eliminate the sanding, you increase the chances of other problems - solvent pop, blushing, wrinkling, etc. If you want to eliminate scuffing between coats, stay with nitro. I'm not trying to sound abrasive (no pun intended) but when you are dealing with professional coatings and go higher up the line into conversion coatings, you cannot keep the same mindset of yesteryear. Not sanding through the edges is tough, and no one is perfect, but if you are breaking the edges before applying your initial coat of lacquer, and spraying the proper mils, you should be able to sand them as easily as the rest of the panel.

From contributor L:
I use MLC products, Krystal and MagnaMax and MagnaKlear. If you shoot Krystal and recoat before an hour passes, it will bite into the previous coat. If you do the same with the Magna series, you have about 1/2 day before you need to sand.

The best advice is to sand between all coats - that is what the instructions require and you will have no recourse if you have a failure if you do not sand between coats.

For your corner burn-through problem, a few suggestions. You can use Mirka soft gold. You still need a light touch. I use 500 grit. The better way to do it is get a full set of Mohawk Blendal sticks. I use to fret anytime I burned through the color, but no more. The Blendal sticks are little color miracles.

From contributor N:
Thanks for the Blendal stick tip. How many sticks are in your set? Do they ever dry up and go bad or do they last indefinitely? I sometimes go to an art store and buy colored markers for my touchups, but that is hit or miss on matching a finish.

From contributor L:
I probably have 20 or so. Need to be prepared for all finishing. Plus I bought the whole kit of wax crayon touchup sticks. About 122 of them. What amazes me is that you can put finish over them even though they are wax - they work great. Plus I have about 20 markers - makes touchup pretty easy.

From contributor T:
It appears Blendal sticks are a dry material that dissolves into lacquer or cv. Is there a product like that for WB finishes?

From contributor M:
I have used nothing but CVs for the last 15 years, and I have never sanded between sealer coats. I spray one coat, let it flash off, spray another coat, then after it is fully dry, I sand and top coat. By sanding after 2 sealer coats, I get little to no burn through on edges. Using this method, I have never had a finish problem.

From contributor N:
What are you using for sealer?

From contributor M:
Currently I am using Chemcraft's Chemlife 24 as a self sealer. It sands real good and has a TR6 rating. But in the past I have used ML Campbell Krystal sealer with KlearVar as a top coat, and also DuraVar. I have also used Gemini products. I have never had a problem with any of those systems.

From contributor N:
Thanks. I was just wondering if you used the CV as a self sealer and how much you thin it to use as a sealer.

From contributor L:
Almost wonder if you should do it the other way around. Shoot the sealer and let it dry, then sand. The sealer is designed to sand easy, so why not take advantage of it? Then shoot your topcoat and after it flashes, shoot the second coat. You would still have to worry about burn through on the color this way, but you have the ease of sanding and that is the time consuming portion of finishing. Spraying is the easy part.

From contributor M:
I don't thin any of my clears. I just spray them full strength. Personally, I would spray 2 coats prior to sanding, regardless if it's a sanding sealer or self sealing top coat. Trying to sand 1 coat of sanding sealer without burning through edges would be difficult, especially on dark colors. That's why I am using Chemlife 24 - it sands as good as a sanding sealer, but has the clarity of a top coat. It also has a 24 hour pot life, so you can seal one day, topcoat the next day without having to clean the gun in between.

From the original questioner:
I really like this conversion varnish. I sprayed two coats back/back, scuffed with a gray Scotchbrite and some 400 paper, then shot two more coats and it looks fabulous. A lot less spraying than what I'm used to with lacquer.

From contributor M:
I would use maroon scotch brite and 320 grit paper. 400 is getting pretty fine.

From the original questioner:
Thanks - I was thinking the same thing. I'll try that next.

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