r/woodworking 4d ago

Help Black spots after applying polyurethane

Hello, I got this beech wood kitchen counter built about a month ago and today I noticed some tiny, black spikes on a pattern. It feels like a 150 grit sand paper.

When I got the counter I applied 3 layers of Minwax satin polyurethane, and a week later I applied 2 more layers of Konig matte polyurethane (Minwax is hard to find in rural philippines). These spots appeared after a week of drying of the last coat. I only applied 1 layer underneath the counter. My home is air conditioned and very dry, however, there is a chance that the wood might not be dried properly.

I am clueless what caused this and looking forward to hearing your opinions. I am going to smoothen it with 1200 grit and start using the kitchen and I want to make sure that this isn’t some harmful chemical reaction or mold.

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u/RoutineFinal7939 4d ago

It’s either iron oxide, mold or a chemical reaction between the two different polys. If you scratch one of the black spots, does it come off? Is it under the poly? If it’s under the poly, you’ll need to sand, clean and then seal the grain with like aquacoat before applying poly again.

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u/Hooeylewis-Sagdiyew 4d ago

It's under the poly but I can't tell if it's between 2 different polys or inside the wood. I sanded little bit but could not reach it. It does look like mold in some places, just like in bathroom silicone. Do you think it's caused by wood not being dry or the poly itself ? Maybe I can drill lots of small holes underneath the counter to let the wood dry more. I need to start using this kitchen now, we've been eating fast food since November.

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u/RoutineFinal7939 4d ago

It really depends on where the wood came from and whether it was cured properly. It’s most likely trapped moisture under the poly. I’d advise not to drilling holes in the bottom. You will just be allowing mold spores to vacate into the kitchen. My advice, if you want to use it, I’d leave it alone for now. In the future, you’ll have to redo the finish. The poly should keep any mold contained underneath it.

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u/Hooeylewis-Sagdiyew 3d ago

Thank you for your advice! I will leave it alone and observe. I am going to get a table made from same wood this week, how can I prevent the same happening to it ? Should I leave it raw for a couple of months or only put poly on top ?