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sanding between coats

Posted by poorowner (My Page) on
Thu, Oct 2, 08 at 3:23

I am building 3-5 coats of Spar varnish and used gloss, so far.

Below is the result of sanding with 2000 grit sand paper, it abraded the high spots and they turn into white specks from the scratches. So far the next coat seem to cover these specks and the next coat of gloss looks nice again.

however I am about to change to satin as it is the final sheen I want and I am wondering if it will cover the sanding marks as nicely as gloss.

Do you think Spar Varnish does not like to be abraded in that case, how to rub out the final coat without causing these white scratch marks? Because I don't recall this problem as much with polyurethane.

Or do I need to let it dry more before sanding?

Follow-Up Postings:

RE: sanding between coats

2000 grit paper is silly for this application. Do you mean 200 (or, more likely, 220) ?

There are two reasons to sand between coats. The most basic is to promote good adhesion, and the other is to eliminate dust nibs and brush marks so that you end up with a smooth, even surface. You've probably achieved the former, but if you intended to level the finish and produce a highly refined, rubout-worthy surface then you have more sanding to do, at least as of when this picture was taken. It's also worth mentioning that there's little point in using satin as the final coat if you're going to rub it out anyhow.

All of that said, I don't have a lot of experience specific to rubbing out spar varnish. I suspect you may be attacking it before its fully cured, and the abrasion is microscopically tearing the surface rather than cleanly cutting it, and that's why the white spots. I suspect you may need to let the final coat cure for a long time -- maybe a couple of weeks -- before rubbing it out.

RE: sanding between coats

How long are you letting it dry between coats? I did a tub apron and some cabinet and bathroom doors with Helmsman spar urethane (not sure if they've changed the formulation, or if you're using something different) about 10 years ago. The stuff bubbles and attracts dust like crazy (stir, not shake! and brush slowly). I was doing this in the summer in a non-airconditioned room, I had to let it dry a good 24 hours not 6-8 hrs as it said on the can to be able to sand between coats. Don't recall what grit I used, but no higher than 220. Just use the satin top coat and don't rub it out at all - an elastic finish like this doesn't rub out well, it just tears. Though if you really wanted to, I guess you could wait a couple of weeks to a month then try - worst case, you'll have to put another coat on.

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