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Clearcoat strategy question


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Mates,

I have been enjoying the benefits of using 2K poly both with regard to cure time and hardness. I have been using Spraymax cans because I do not have the facilities for a spray booth. 

The issue comes in due to the limited shelf life of the product, which is about 48 hours after the hardener is introduced (and that may be too generous). When I sand and flatten, I have always needed one last thin coat to fill any issues that need correction and to give a nice final gloss coat. However, at about $ 20 a can, I don't want to open a new can for that one light spray. It also seems rather wasteful. I never have a simultaneous project available to otherwise make use of it.  Therefore, I wind up using a 1K poly for the final coat. The end result is that the top coat does not have the hardness of the lower layers, significantly diminishing the benefit of the product. 

Maybe I need a new methodology. Or maybe I should just suck up the cost and tolerate the wastefulness of using 5-10% of the second can. 

Any thoughts on this dilemma are greatly appreciated. 

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I've used 2k poly a couple of times, with a spray gun though. But it doesn't make a big difference.

The way I was taught to use that was to spray a base coat thick enough to be sanded down to the bottom of any pits and orange peel craters. To achieve a thick enough layer it had to be sprayed "wet on wet" with some 15-30 minutes intervals. The main thing was not to let the poly create a skin. If that happened it had to be left to cure.

The first layer is about filling any voids to get a level uniform surface. Should that be sanded through you'd have to apply another coat and let it cure, then sand flush.

But if everything goes like it should you'll have a nice satin base for the second layer. Again, wet on wet trying to avoid runs as well as orange peel - for what I've understood the latter is mostly a matter of mismatching distance, pressure and temperature so a test spray is recommendable every time the conditions change. Which means every time...

Anyhow, you should end up with a rather thick new layer of 2k all around the guitar, including edges.

Now it's time to start level sanding to the bottoms of any orange peel, hoping that they aren't too deep. If you sand through the second coat you'll have to start over as the transition to the first coat will show when buffed. But hopefully all goes well and you end up with a nice level matte surface. Then "all" you'll have to do is go through the grits up to at least 2000, changing the sanding direction by 45 degrees every time you change to a finer paper. Some do it dry to prevent wood from swelling and maybe for seeing when marks of the previous grit disappear but I prefer wet. Wiping the slurry off dries the surface enough for examining scratches.

After 2000 grit you can either move to a medium coarse cutting compound and then to a finer, or you can use sandpapers as fine as you can find. 6000 gives an almost shiny surface with a milky hue. To remove that a fine grade cutting compound will do the trick. If you have a buffer machine you can start buffing after 2000 grit.

I've used good old elbow grease for all the steps but a random orbital may help on flat surfaces for leveling. That said, wet paper and an electric tool may not be a good combination and dry can cause the friction to melt the poly.

As you can see, no "gloss coat" is needed.

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Thank you for the very detailed explanation of the process. Very helpful.

What I refer to as a gloss coat is the same as your second layer, which I presume would be thinner than the first layer/base coat, and would not require anything close to a full can. I guess your advice might be to just use what is needed of a new can and discard the rest, which I suppose is not that tragic in the scheme of things. 

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41 minutes ago, RVA said:

What I refer to as a gloss coat is the same as your second layer, which I presume would be thinner than the first layer/base coat,

My second layer should be much thicker than the first one! Plenty enough to allow sanding level with the bottoms of any orange peel craters or other dips.

The first layer is basically a filler and most of it will be sanded away, leaving the thinnest possible layer so that there's no bare wood.

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