Problems Sanding Conversion Varnish

      It could just be a matter of timing. But here are other ideas as well. June 18, 2013

We have been spraying Sherwood Water-White CV (solvent based) for a while, but we aren't terribly impressed with the curing time as the temp drops below 70 in the shop. We keep the shop over 60 overnight, so the wood isn't starting out cold and neither is the finish. We also heat them with a heater to 80 or so as they cure.

The curing just takes an eon. Today, it's nearly 80 in the shop (Charleston SC) in December, and although they seem to dry reasonably quickly and they seem to halfway powder okay, they look awful when I spray them with their second (final) coat. They cured for at least an hour and a half before I started sanding them. Most of them, 2 hours, which should be more than sufficient with CV.

I'm thinning both the first and second coat with butyl acetate around 10-15% depending on how it's flowing out that day. Spraying 3-4 wet mils. Catalyzing it with the specified catalyst at 3%. Spraying dull rubbed sheen. The final product, if I can get the doors sanded well, looks beautiful. But I have to spend 3X the amount of time sanding to get it to look right. Anyone else have this issue?

Forum Responses
(Finishing Forum)
From contributor N:
What looks bad? My first thought went to cleanliness, but thought that's crazy (as though you weren't getting them clean enough after sanding). I use medium rubbed effect 99% of the time, which is neither here nor there, I suppose. So, when the second/final coat look so bad, what are you doing? Sanding and shooting a third?

I have very limited experience in spray equipment. I bought my first one (Apollo HVLP turbine) and never another. I added a 2 qt pot along the way, but that's been the only change to my setup. What are you using - turbine HVLP? Conversion gun? Conventional? My suggestion would be to change reducers on your next go 'round, or mix up a small amount to play with on something that doesn't matter. Yes, that cussed stuff is expensive, but maybe it would save a load of headache.

From the original questioner:
We were originally thinning this with MAK (methyl amyl ketone) based on the suggestions of our rep. It was even slower. We're using an AA gun. It looks like it was only halfway sanded. I've been doing wood finishes for 8 years now and I know what the reasonable amount of effort is that should go into sanding a door flat. The finish, after 2 hours of curing at 70 degrees, still often has a leathery feel (you get some powder) and you don't get it cutting nice, flat, and dry. It's still somewhat soft. It's that last little bit of hardening it doesn't seem to want to do. It looks like a poorly sanded door, or a door sanded too early. Looks bumpy.

From contributor J:
Have you ever considered using a post cat sealer? I use Becker products, and their Careseal builds great, dries in 30 minutes to sand, and sands great. I know it is problematic if you want to seal and top coat small batches. You have two products to mix and spray. We seal everything before we start spraying the topcoat. One seal coat and one top coat is plenty of build. I would never consider not using the sealer. Once in a while we need to two coat with conversion varnish a few parts that got missed when we sprayed the sealer, and I hear your pain.

I think the conversion varnish doesn't fully cure in the hour that it is given to dry, and is re-wetted when the second coat is applied, so it doesn't lay down flat and tight like when spraying over sealer. In my experience, it will tighten up as it loses volume while the solvents evaporate over the course of a day or so.

Is the material meant to be thinned before spraying?. The Becker CV is not meant to be thinned, but is catalyzed at 12 - 13%. Becker has a slow and a fast catalyst. We use the fast for CV when it gets really cold, but continue to use the slow for the sealer.

From contributor A:
You're sanding too soon. If you sand before the film has settled, the scratches from 320 (280) paper open up as the solvents flash off.

From contributor A:
Don't spray with a heavy hand. Use fast reducer (xylene). Soft sand (280,320).

From contributor Z:
I spray Valspar CV, but find it needs a good 4 or 5 hours before it will sand really well. I usually seal one day, scuff and topcoat the second day.

From contributor E:
Butyl acetate is a medium evaporating solvent that works great in warmer temps but when the weather turns cold can be too slow and act like a retarder. Ditch it and use a faster solvent.

From the original questioner:
Thanks for the responses. I agree with the above comment about spraying with a light hand. I do spray a bit heavy handed, usually around 4 wet mils according to my mil gauge. I find that spraying a skinny 3 wet mils has helped the situation some. As for using a post-cat sealer, the only post-cat sealer that Sherwin Williams has offered me that uses the same catalyst is their vinyl sealer, which I do not care for. It doesn't sand any easier than the CV does.

From contributor C:
Too soon. I always do overnight before sanding. Also, this time of year thin it 15% with lacquer thinner when used as a sealer. I plan my finishing so there is always something drying, something to be sanded, and something to be sprayed.

From contributor R:
I use 2k urethane and sand in less than an hour with no problems. (Powders like good old lacquer sanding sealer.)

From the original questioner:
I would love to spray 2K urethane, but I'm not interested in having to deal with isocyanates. Most of what I read about them is that they absorb through the skin more than what is in most CV's, and that you really need to wear a body suit. My shop hits 99-102 during warmer days in the summer, and I have no interest in that. The mask itself is like having a sweaty leach strapped to your face.

That, and the typical 30 minute pot life would drive me nuts. I've sprayed a few gallons of 2K poly in the past (ICA and MLC) and they are nice products, but the pot life and isocyanate issues bother me.

From contributor B:
What gun are you using? You mentioned AA. Is it a pump?

From the original questioner:
CA Tech with a Lynx gun. Air pressure (dynamic) at 18-20, fluid pressure at 30. Same setup I've sprayed most other clears at with very good results.

From contributor T:
I saw that you add 3% cat. I use MLC CV at 10% and don't have any problems. I always box coat my first coat and now that it's cold, around 55 to 60 degrees, I wait about 4 hours. With your conditions less than 2 hours should be long enough. Can you add more cat? Have you had this problem from the start? I would check to see if you have any water in your compressor also. Every time I have a problem, it seems to be water related.

From the original questioner:
Campbell's is 10% because they dilute it for easy math. Sherwin-William's catalyst ratio varies from one to the next. Some of their whites are a 5% catalyst (same catalyst). Never add more catalyst to get it to harden. This isn't like polyester/bondo where you can add a bunch more hardener to make it work better - you will end up with acid weeping out on your cabinets/doors in time.

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