Hi, I am apply oil based poly (thinned about 30-40%) to the surface of my wood project and end up with a lot of very tiny (not quite microscopic, but you need to be close to the wood to see them) areas where the poly doesn't settle very well. I don't know how to describe it other than it looks like little microfibre parts of the grain that the poly doesn't adhere to. I have several coats of poly applied and have lightly sanded with 400, then 600. This last time I tried 22, 400, and 600 before each coat and still get spotty coverage. I am cleaning the surface very well prior to applying poly. Is that the grain of the wood causing that or are those tiny scratches? Would it help to not lightly sand before the next coat and start building up the poly in those areas? I am new to woodwork finishing so may have don't know if this is normal or if I did something incorrectly along the way. Any suggestion? Thanks!
Poly not covering evenly
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There are a couple of things that come to mind. How long did you let the stain dry before applying the poly? If you didn't let it dry overnight then the stain may be reacting to the poly. Another problem you may be having is thinning the poly. The only time I thin poly is to spray it and I thin it as little as possible. Thinning 30 to 40 percent its too much thinner and not enough substance. It would eventually cover but would take many many coats. As far as the scratches I normally sand with 220 grit between coats. I think the only reason you are getting scratches with the finer paper is because the finish is too thin. Try a spot without thinning it at all and see how it does.
Thanks Steve. No stain used. Thinning poly does seem to reduce the air bubbles, but what you said makes sense. I will try spots w/o thinning, or change the thinning ratio. This is my first time doing a fine high gloss finish and it is turning out to be try and adjust. You think those are micro scratches?
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The top of the cedar chest is made of solid walnut. I filled the grain with a pastewood grain filler and finished it with I think Dupont furniture lacquer. It was what I was using in 1975 when I built it. I have since quit using it. It seem to get too hard and tends to crack if you get it a little thick. I would probably finish it with a pre-catalyzed lacquer if refinished it but you could put that type of finish on with most any gloss finish. It's just less labor if you have the equipment to spray the finish. That way there isn't so much hand work rubbing out the brush marks.
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How did you apply the poly? How long did you let each application dry? When you said you cleaned the area prior to applying poly...what did you use to do that?
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I am waiting ten to twelve hours between applying poly coats. Before the first coat I wiped it down with a clean rag, then a slightly damp rag, then vacuumed it. I am applying the poly with a brush. Regardless of whether those are micro scratches or the wood grain, now I'm wondering if they were there prior to applying the first coat of poly and got filled up with packed down dust and the dust sort of disolved when it absorbed that first coat of poly making the scratches/grain more pronounced. Is that possible? I could not see or feel any scratches on the wood before the poly. It was perfectly smooth.
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It's possible there were scratches in the wood you couldn't see until you started finishing it however the dust wouldn't dissolve into the finish. As much as you cleaned it off there shouldn't have been any dust left but I'm wondering if there was moisture left from wiping it off. If you get water under the finish it can cause some white or milky looking spots and still have a smooth level finish. Usually when I wipe water on a finish I will dry it off with paper towels or use a hair dryer before I put a finish over it. Really I don't like to use water on a finish anyway. Water seems to hide in cracks and crevasses and when you spray a finish over it a bead of water runs out. I prefer to clean sanding dust off with compressed air.
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You got troubles if a thinned coat of oil poy won't stick. Am I right that your problem is bare spots in the finish after it dries? If so you have some kind of contamination in the wood (wax, silicone furniture polish is the worst) Cheap steel wool can also interfere. If that is your problem, you have to seal the contamination in the wood under your top coat. Sorry.
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Using compressed air will distribute dust everywhere. If done where the piece is being finished, it will be subject to what is in the air. Dust stays airborne quite a long time.
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I used part of a clean cotton T-shirt as a rag and moistened it very lightly with water. The rag was damp enough to remove dust but not wet enough to leave moisture on the wood (it almost self-dryed, but I went over it with another clean dry rag).
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Since you think the surface was free of moisture then I bet the poly you were using didn't like being thinned that much. You might try buying some wipe on poly. It is already formulated to be used thin.
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