Whether to Thin a Conversion-Varnish Seal Coat

A discussion of seal coat application and performance with conversion varnishes. September 16, 2008

I'm a shift leader in a busy fixture manufacturing facility in Tulsa. I've been trying to educate myself on the product we spray so much, which is Valspar Conversion Varnish. We have a great facility, great equipment, and good people including a rep from our local supply house that sells us the Valspar.

I like the product, but in my viewings of threads here I've noticed several posts recommending that the seal coats should be thinned up to 20% to promote adhesion qualities that CV offers.

We use uncut CV to seal over a variety of woods (oak, ash, cherry, mahogony, tay, bubinga, and more). When I asked our rep whether we could cut our sealcoats to help adhesion and save our company some pretty serious money over time, he said it was unnecessary. In fact he stated that our company switched from using vinyl sealers to CV and we should just apply the CV as a sealer straight out of the barrels after proper catylization.

I've read some postings on WOODWEB about these reps, and I would just like to hear from as many industry pro's what they think. For now we continue doing as our salesman suggests. I really like this sight, and have learned a lot from contributors.

Forum Responses
(Finishing Forum)
From contributor L:
Some of the reasons to thin out your seal coat are better penetration, ease of spraying and ease of the initial sanding of the sealer. Some of the reasons not to thin out your seal coat are that it makes it less prone to sand through because you have more material. You build your film thickness quicker so you need fewer coats. But it also makes it harder to sand initially because the nibs are thicker and harder because of more material.

From contributor F:
If you thin it out you are only lowering the solids content. As far as I know, this will have little to do with adhesion. I think the belief that thinning promotes adhesion stems from the same notion that using a separate sealer is necessary to "seal" the wood before applying a finish.

Some finishes such as NC or CAB lacquer benefit from using a separate vinyl sealer but that is because of the limitations of those top coats, not because of any magical properties of the sealer.

CV does not have these limitations so in most cases it does not need a separate sealer. Or possibly it comes from the idea that a thinner seal coat can soak into the wood more. I don't hold much stock in either idea. A lower solids seal coat just has less film on the surface when dry.

Thinning it does not change the size of the cross linked resins nor allow them to "soak" farther into the wood. A properly prepared surface and properly applied stains have far more to do with good adhesion than the solids content of the first seal coat.

From contributor M:
More important than thinning the CV, which if it's Resistovar is already fairly thin, is the pre-sanding. Be very careful to not sand with too fine of a grit. The CV needs something to bond to. P120 on maple is about as fine as you should go. P220 on softer woods such as Alder will work.

I thin my CV for better flow-out since I live in a desert. I think thinning the first coat is a good idea but I'm not convinced it's at all mandatory and if you're spraying it on with an Airmix it almost certainly isn't necessary.

From contributor L:
Thinning your seal coat is unnecessary if you are getting the "look" you want. In some cases thinning helps in flow and level, especially in the summer, but only if they are thinned with the proper material. Lacquer thinner should not be used. A thinner is better designed for conversion varnish. I guess the bottom line question to ask yourself is, "what would I gain by thinning my seal coat?" If you can't answer this question, I would steer away from thinning.

Thanks to everyone for their input on this. You guys brought up some good points. The seal coat is definitely easier to sand, but with all the stains we apply the lighter coat would probably just cause more problems with burn-through sanding. Thanks again for the responses.

From contributor O:
Valspar CV is designed to use a premium Lacquer Thinner, Butyl Acetate or Acetone for reduction, not Xylene.

From contributor R:
Nobody mentioned here that thinning your CV will help eliminate bubbles in your seal coat when spraying open grained woods. If you’re not having those problems, why change.

From the original questioner:
"We have tried thinning our seal coats with the special thinner our supply house made for our company. The sanding is definitely a lot easier. Without thinning it almost feels like there is a lot of trash in the finish, although we blow everything down prior to stain or seal regardless of whether the seal coat's cut or not. It makes sanders feel they need to sand extra hard and sometimes that causes burn-throughs especially on edges. But that wasn't my only concern. I also wondered about adhesion.

In the "high speed production finishing" area on this homepage, a thread titled "Problems spraying CV" has a comment about adhesion from contributor D that got me wondering if we weren't getting the most out of what we spray. The comment is about halfway into the thread and stated that "in reality you must thin your CV", but that dealers won’t say it (especially in print) because of HAPS regulations. He said he thins all of his CV so that it "sprays better" and "forces it to penetrate better so you get good adhesion". He goes on to say that "the key to the first coat is penetration". That is what got me scratching my head.

As stated earlier, I am learning more about the product we spray so much of. I trust the advice of professional finishers more than I would a salesman, but there are finishers and there are finishers, the same with salesmen.

From contributor R:
I've only sprayed SW CV. However, I always needed to thin my first coat. I apply it at the max wet mil thickness though. If I didn't it was too hard to sand and also had trouble with bubbles, solvent pop, etc. I also buy into the adhesion a little bit too. I wouldn't put too much weight behind it, but some.

From contributor L:
Valspar should be able to help you address your concerns about sanding and adhesion. I do believe that white-wood sanding contributes (or diminishes) adhesion of any coating, especially on harder species, like maple.

According to your last comments, you have thinned your sealer with satisfactory results, because you were finding "trash" in your sealer and this made it harder to sand (please tell me if I got this wrong).

It has been my experience of the opposite, that thinning increased the grain-raise and made things harder to sand. It is also possible that when you blow off your substrate, you are creating static electricity that is drawing dust to your work during finishing.

How hard are the guys sanding? Today's CV's do not require heavy sanding, just a good scuff to "knock down the sheen". If you have to sand a lot to get a smooth surface, then you have an issue getting your sealer to lay down correctly. This could be from the application of the sealer or going into the oven before it has had a chance to flash off. Other factors could be contributing to this, but these are the most common. The coating is almost never the problem.

To test this, finish a door with your current system (thinned sealer, blow-off, etc.) and finish one using un-thinned sealer, hand wiping the door to remove dust and see which one gives you a better surface to sand.

FYI on thinning: Manufactures don't recommend thinning higher performance products since most are formulated to be sprayed right out of the drum without adjustment. You, being an industrial user of coatings are exempt from VOC laws, but not HAPS rules. The more you thin, the more you will have to report.