Spray Quality Issues, Sandpaper Choice, and Inter-Coat Adhesion

      A finisher has concerns about both the off-the-gun quality of his base coat, and the possible problems created by his sandpaper selection. Colleagues straighten him out on both counts. March 30, 2008

Question
I am working on a painted/glazed project. All SW products, white vinyl sealer, alkyd glaze, pre-cat topcoat. I cannot seem to get the white vinyl to lay out just right off the gun with no overspray. I tried sanding a sample with 2000 grit Norton Sandwet waterproof sandpaper, and then glazing. The 2000 grit was coarse enough to denib what I needed it to. I then glazed the sample and it did not have deep enough scratches to cause a problem with glaze lodging in the scratches. I have never used this type of paper before. Could it cause fisheyes, etc with the topcoat?

Forum Responses
(Finishing Forum)
From contributor J:
I've never had a problem with Nortons. I like their Black Ice the best and have used it a lot for auto paint prep, which it's designed for. That demands a clean product.



From the original questioner:
Sorry, I should have included the complete schedule.

- 2-3 coats tinted vinyl sealer
- (hopefully sand with 2000)
- glaze with van dyke brown alkyd base glaze
- topcoat with 2 coats pre-cat

The paper I am looking to use is black in color, but I don't think it's called Black Ice. Not sure if that is the same thing.



From contributor R:
Using such a fine sandpaper could cause adhesion problems worse than any kind of contamination could possibly create. Don't try to cure faulty spraying techniques with sandpaper. Discover what is causing the spray problems and correct them first.

I've sprayed SW white vinyl on many projects with fine results. What kind of spray gun are you using? Is it hooked up to a pressure pot? What PSI range are you spraying in? Have you used some retarder in the mixture? How much do you thin the product? Are the gun parts absolutely clean? If not, that's the cause of many a spray problem, overspray being one.

Let's solve the spraying problems first and not worry about the sandpaper. Once you get the spraying end of it down, you can use 220-320 grey silicone carbide papers by most any manufacturer. I happen to like 3M, but that's just my preference.



From contributor G:
I have used that finishing schedule several times but not with 2000 paper. That is way too fine. 320 or 0000 steel wool is plenty. 2000 paper is for polishing, not scuffing, a surface for an application of another finish. As said, this will give you adhesion problems. You need to have some tooth for the next stage to stick to. A light sanding with 320 will not, or should not, leave scratches deep enough to cause problems with your glaze. But adding glaze will leave a slight tint and change the color of the white. Is this what you are trying to avoid? Rub the glaze harder and faster or add a little naphtha to your rag to get a cleaner non-glazed surface.


From the original questioner:
Thanks. I am spraying using a Turbinaire HVLP (0.75 needle and #2 aircap I believe). I don't know why I am having so much trouble with it. On moldings or columns, or something else small, it's not a problem to keep a wet edge, but when doing a larger door or face frame, by the time you get back around, it's dry already. I know retarder would help this, but it seems to me that you have to go painfully slow to lay out enough material to flow out completely/break the surface tension.

I have tried thinning, as well as different turbine speeds, different material flow rates, and I feel like I am using the proper needle and aircap per the chart given with the gun. I might have had this trouble with clears before and not even known it. This solid color lacquer finishing is a completely different beast.

About the color of the glaze, changing the overall color of the surface a little bit is desirable in this case. I am just trying to avoid sanding scratches for the glaze to lodge in.

I didn't even consider losing tooth for the topcoat with the 2000 grit. I'm glad I didn't start that! I tried 320 on a sample and it left scratches that were too deep. I had a bit more success with 400, followed by a 0000 synthetic steel wool. Is this okay to topcoat? Would 800 be too fine to use?



From contributor J:
I don't know if this helps, but I sand with 600 grit and it works fine for my lacquer topcoats. If I use 400 or coarser, I have scratches show. So 600 for me! And that's 400 on the Cami scale. Not the "P" number scale which would be more like "P1000" grit.


From contributor P:
.75 is a pretty small needle. Could be that it's atomizing the finish so much that it's dry before it hits the wood. I'd try bumping up to at least a 1.0. I have the Turbinaire, too. I spray most topcoats with a 1.5 needle and #2 aircap. Your finish manufacturer should be able to tell you what the best needle/cap combo is.


From contributor S:
Ditch the turbine (anything but) and the 2000 grit (220, 240, 320) paper and your troubles will vanish.


From contributor T:
Correct me if I'm wrong, but I think the topcoat will make a chemical bond with the vinyl and itself, therefore adhesion is a non-issue.


From contributor R:
What, you think the pre-cat will melt into the vinyl? Don't know, but I would not chance it.


From contributor C:
Yes, the pre-cat will melt into the vinyl, if the vinyl is not catalyzed. But you do take a chance that if too heavy a coat is applied, or if you're using retarder at all, that it will potentially lift the vinyl. Remember in the initial stages of application the pre-cat is like normal nitro lacquer; not until the bulk of the solvents evaporate and the oxidation starts does the alkyd/amino start crosslinking and become the catalyzed finish. That's the reason they have a recoat window, and once reached, you can no longer spray unless you sand or abrade.

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