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thefluidmechanic · 1 year

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Spar Varnish Spotting Issue [Solved]

I finishing a Walnut knee-wall, using Helmsman's Spar Urethane thinned in a roughly 3:1 ratio with mineral spirits.

The first two coats went fine, first using a brush and then a foam roller, with light sanding and cleaning in-between coats.

During the third coat, it started to rain outside and 1 forgot my windows were open, so the humidity was relatively high. I closed the windows, as soon as I realized, and turned on my dehumidifier.

After this third coat, I got these small impressions, which I lightly sanded and then added a fourth coat, thinking it would fill in the spots.

Here is the result after the fourth coat; it was worse than the third coat.

When I rubbed the wood with mineral spirits, the spots seem to disappear until the solvent evaporates and then it looks the same.

I experimented, in two small spots, with more varnish, diluted, undiluted, and with polyurethane.

A thick coat of undiluted spar varnish did the trick!

I also ensured the humidity was low (<50%)

#woodworking #wood finish

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