Problems with Conversion Varnish Sanding Dust Residue

Finishers discuss how to get pieces clean between the sanding and topcoat steps. December 2, 2009

I have been finishing some oak and using CV. I am not sure what steps I am missing. After a seal coat I sanded everything with a 320 grit sanding pad. Last time I used CV I sprayed the finish coat only to find that there were white spots in the grain from the sanding dust. This time I tried to blow off the dust and use a paintbrush to help clear the open grain in the oak.

This was taking far too long and getting into the negative as far as profit, so I used mineral spirits, blew off and wiped down about 1700 ft of trim before shooting the finish coat at 3 to 4 mill thick. The finish turned out real nice, but I am there to make money for the company and there is something wrong with my method. I kind of like the CV but I have to get the speed up to what I can do with a pre-cat lacquer, or at least close.

Forum Responses
(Finishing Forum)
From contributor M:
How long did you let the sealer coat cure before topcoating? Maybe it cured too long and did not melt from the topcoat? I personally have never had that problem. I always used Duravar and KlearVar for years and never experienced what you are talking about. The only time I have ever seen that happen was when I used a Gemini CV. I also blow everything off really well with compressed air (about 150psi), and wipe everything down with a clean rag, then reblow it off before topping.

From contributor R:
This is truly an issue for your employer to consider when bidding a finishing job.

I'm sure the cabinetmakers blow off the raw wood dust leftover by sanding, otherwise the finish is going to end up being rough. You have to blow out the sealer sanding dust otherwise it will be locked in the grain by the conversion varnish. Unlike a lacquer, which will melt the dust, I have found that unless I remove most of the sealer powder when using a conversion varnish it will end up showing through the topcoat.

Show your employer the difference between the way you want to do it and the way they would like it done and let them make the decision.

On a side note, you may try to crank up the air pressure when you blow off the dust and if necessary, try a tac rag along with the compressed air.

From the original questioner:
Thank you. I let the sealer cure for about 40 minutes before sanding and then it sat on the racks overnight. I am under the impression CV sealer and topcoat do not melt together like a pre-cat lacquer. I have heard that one might use a pre-cat sealer before a CV topcoat, but I am not sure of this.

I used at least 120-150 PSI to blow off the dust, but found it did not work well, then we used the mineral spirit wipe down. That worked, however it was time consuming. We haven't used much CV but I would like to know what I can do to cut time and cost.

From contributor M:
Maybe you could try a vacuum with a brush attachment to get most of the dust off initially. Maybe you are just blowing the dust into the open grain of the oak, making it more difficult to remove. Also, be careful about using a pre-cat under a conversion varnish - you can end up with surface tension issues when you put a harder finish on top of a softer finish.

From contributor S:
Try blowing the sanding dust off the piece right after sanding. It's likely that by racking the items overnight with the dust you are getting a slight biting action as the sealer and its dust continue to cure, making it much harder to blow off.

If it still happens try changing your sanding technique. You may be sort of grinding the coating into the pores. Make sure you are using a no-fill type and don't repeatedly sand over the same area without blowing off the dust periodically.

From contributor E:
You might try using a tack cloth between coats.

From contributor C:
I've been spraying CV for 20 years and here's what you do. Rag in one hand and blow gun in the other. Also, there is this duster mitt with many little fingers on it. Found it at HD and that simply is the best dust remover I have ever used.

From contributor J:
I use a hand broom to clean the dust out of the pores, then blow off with a blow gun. If you are sanding then letting it sit on the rack overnight before top coating, the dust is continuing to cure. You should be sanding the same day you topcoat. Don't scuff one day then topcoat the next.

If this and a thorough cleaning of the dust out of the grain is not profitable time wise in your situation, then CV may not be for you. I see a lot of shops who use CV for jobs that would be fine with pre-cat on them. You have to ask yourself if the CV is really warranted for what you are doing and if it is, you will have to find a way to either live with the time it takes to do correctly or find other ways to improve production time.

From the original questioner:
Thanks for the info. It sounds like racking the substrate overnight after sanding could cause some difficulty. Thing is I did use CV on a job not long ago and did the topcoat the same day with the same problem - white spots in the open pores of the material.

I am not sure when we will use CV again but I have to say as a finisher I love the hardness of it and if we can find a way to up the production speed I would like to use it more often. I had good luck with it this time wiping it down with spirits. The overall finish is up to our standards, however I will use what you all are suggesting and see how it can up the speed.

From contributor M:
You might want to look into trying Duravar (ML Campbell). It is a post cat finish that acts a lot like a nitrocellulose. I don't think you would have any problems with it. It is probably the most foolproof post-cat finish I have ever used.

From the original questioner:
What is nitrocellulose? Is it a water base?

From contributor M:
I am not much on the chemistry end of finishes, so maybe others can give you a better explanation of what a nitrocellulose lacquer is. But one of the characteristics of a nitrocellulose lacquer is it will burn into itself, so you won't have the problem of dust not melting into the finish. Duravar always seemed to display a lot of the characteristics of a nitrocellulose lacquer, with the hardness and durability of a post-cat finish.

From contributor J:
If you look up nitrocellulose on Wikipedia, it talks about the chemical itself being used to make flash paper or gun wadding.

But for the purposes of finishing, it generally refers to nitrocellulose lacquer, which pretty much just means good old straight, non-catalyzed lacquer. Confusion comes when the chemical name is used as the slang term for lacquer. The actual chemical is used in other types of finishes as well. Pre-cats have some degree of nitro in them, some more than others, and even some post-cats have it.

The Duravar that contributor M mentions behaving much like nitro does so because it has nitrocellulose in it. This is why MLC got called out years ago for calling the product a conversion varnish and now refers to it as catalyzed lacquer. A true CV should have little to none in it.

From contributor J:
Sorry, I forgot to mention that nitrocellulose is made from cotton.

From the original questioner:
Thanks. I am starting to get the picture. I sure like the CV. We had a rack overload and found about 300 ft of trim on the floor with the rack on top. Other than a couple of gouges, there was no sign of damage. That stuff is hard and I love it. We use a lot of Valspar pre-cat and I am good to go with that method, just have to stir it often.