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I'm finishing a custom mahogany wood sill for an interior wall opening and have stained and finished it-- lightly sanding with 220 between four polyurethane coats.

In the first two coats a couple bubbles and particles dried into the finish, but light sanding managed those. The third coat had two small imperfections which I lightly sanded down leaving a whitish haze that should have disappeared in the next coat. However, they didn't. They are not major imperfections and only can be seen when light reflects in just the right way, but I would like input on the best fix for these areas.

Will more poly coats work or do I have to get down to the sanded level and fix it there?

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Unfortunately it sounds like you will need to sand out the areas that are affected. If it has a coat over it, there's nothing you can do to retroactively that will fix the problem because it's effectively shielded by the top layer of finish. I think your problem might have been too course a grit on your sandpaper. I would use no courser than 280 or 320 for a poly finish because unlike a lacquer or conversion varnish (which have harsh solvents that actually melt the surface of the previous layer) poly has to float out over whatever is there. Good Luck!

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  • Poly is unfortunately a pain in this manner. When you do sand like paper mentioned make sure you use a very light grit, I prefer 0000 steel wool actually. Make sure to clean well afterward as well. Start with compressed air to clean out the grooves made by sanding (wiping can push the dust in) then lightly dust it with a damp rag.
    – James
    Nov 5, 2014 at 18:03

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