Eliminating Scratches in Glaze

      Stubborn sanding scratches appear when a piece has been glazed... can this be avoided?January 6, 2005

The way I understand the glazing process is this. 1. stain, 2. seal, 3. sand, 4. glaze, 5. topcoat. I use a vinyl sealer with SW conversion varnish topcoat. When I sand the sealed and stained cabinet doors prior to glazing, the dark glazes highlight scratches that are invariably present on the product, especially with darker glazes and lighter wood. How do I eliminate this problem? Is it possible to not sand the sealer prior to glazing?

Forum Responses
From contributor D:
I think your best bet would be to apply a thinned topcoat after sealer sanding and then apply your glaze.

From contributor T:
After the first seal coat I generally use 240 (tri-mite fre-cut by 3-m) to knock off the nipples, using no more pressure than the weight of my hand. No scratches.

From contributor S:
Your glaze is sitting in the sanding scratches of your sealer coat and that is giving you a dirty look which you do not want (but there are projects where you may want this look). After scuffing your sealer coat, apply a very thinned-out coat of more sealer.

I am assuming that because you are using SW conversion varnish, you are using a vinyl sealer.

Just make sure to get all your glazing and locking in within several hours of the application of your sealer coat and you will not have to worry about scuffing as a way of insuring intercoat adhesion. Lock in the glaze with more vinyl sealer.

While the SW vinyl sealer in your schedule is a catalyzed product, if you are creating that vinyl-glaze-vinyl sandwich all within a few hours, then scuffing is not as needed as if you were waiting overnight. By that time, the sealer has cured way too much and scuffing does become necessary.

From contributor Z:
First of all, skip the vinyl sealer (unnecessary), use same product reduced 20-25%. Second, 220-240 to rough, especially if stained. Use 320 paper after sealer, then glaze, wipe off, wait overnight, then seal with 10% reduced, sand with 400 paper and apply finish coat, reduced or not.

From contributor K:
Standard S66 glazes from Sherwin Williams will fail when used in conjunction with their vinyl sealers and conversion varnishes, regardless of recoat times or what combination of sandwich you use. This has been reproduced and acknowledged by them, and it amazes me that they don't make this known. Our local supplier has reformulated the glaze on their own to eliminate this problem. We buy our product from the Seattle Industrial coatings store.

From contributor R:
I'm wondering if you are trying to stain or tone the cabinetry using a glaze? Glaze used like a stain may cause finish failure. It has to have a place to lodge and can't be applied to a perfectly smooth surface. We use glaze to lodge in the sanding scratches. It's important to have a linticular scratch pattern.

You would be better served by spraying the color on in a lacquer compatible finish. If you must use glaze, then closer attention is required to the scratch pattern.

From contributor B:
Stain, NC sanding sealer, sand grain pop with 320, seal with thin vinyl, scuff sand with 0000 steel wool. The steel wool will give enough bite for the glaze without visible scratches. Seal again with vinyl, then topcoat CV. That's how we do it. We use Valspar and Frazee products.

From contributor D:
I would strongly suggest staying away from the NC sealer method. Although contributor B has used this method and apparently gotten away with it, you are setting yourself up for finish failure down the road. You cannot apply a N/C sealer under CV without some kind of problems at some point. Trust me on this one!

And contributor B, please do not take offense to this as a knock at you. I have seen it happen in more than one instance to the point of putting some very big manufacturers out of business. N/C finishes are formulated with plasticizers that will migrate out of the coating in time and if you lock it in with a catalyzed finish, it will lead to cracking.

From the original questioner:
What is a N/C finish?

From contributor B:
NC= Nitro cellulose (regular old lacquer).

Are you saying that the plasticizers will migrate through vinyl sealer?

From contributor S:
Vinyl sealers are one thing and n/c sealers quite another. You will be okay if you're adding catalyst to the vinyl sealer, and if you're not, you'll probably have finish failure soon. Plasticisors help coatings flex with expansion and contraction.

From contributor D:
Let me give you an example: remember the old vinyl dashboards on cars? Well, they were made with a vinyl and plasticizer formulation. The process was sped up considerably with the sun, but basically they would all crack in time. If you look at an old lacquered finish on a table that was finished 30-50 years ago, they all crack. The problem with locking the plasticizer under a cat'd finish is that the cat'd finish will not flex with the plasticized undercoat and could lead to problems as soon as 2-3 years. As I've stated in the past, sometimes you will not hear about a problem - you just won't see the repeat business. So if a potential problem can be avoided, it should be.

From contributor O:
Another thought... NC sealers typically contain zinc stearates which aid sanding. Most post catalyzed lacquers and conversion varnishes use an acid based catalyst. If an acid cure topcoat is put on top of this type of sealer, a chemical reaction takes place between the zinc and acid, causing blooming, and a haze forms in the coating after the finish is fully cured. This haze may not appear for weeks or even months.

From contributor D:
Excellent point, contributor O.

From contributor S:
So what you're trying to say is that post catalyzed coatings have no plasticizers in them and this is the reason for failure of finishes from losing their flexibility?

From contributor D:
No, I am not saying that at all! True, post-cat'd finishes do not have chemical plasticizers. They are formulated with resins, alkyds, ureas, and melamines that stay in the film and are flexible in themselves. A chemical plasticizer will migrate out over time. The time depends on too many variables to list.

From contributor B:
I like your dashboard example. Let me see if I have this straight: The lacquer loses the plasticizer over time, so when it shrinks, it no longer can stretch, so it cracks, pulling the CV topcoat with it? Now if you are only using one coat of lacquer as a sanding sealer, it would be mechanically bonded to the wood pretty well, and cracking would be much less likely than a good heavy build of lacquer with a CV topcoat. Is that the basic physics? Think I'll start using CV as a sanding sealer, though.

From contributor D:
Contributor B, yes to your question. And I think you are doing the right thing by using the CV as the sealer.

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