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I'm building a desktop using oak wood. I used tongue and groove joints to join four planks together, gluing the planks along the joints with Titebond III, but there were some small gaps between the planks because some of the planks weren't perfectly square. I added some wood filler to fill the gaps and let it sit overnight to dry. The filler filled the gaps well, but the excess added some dark spots to the wood that make it look dirty. I'm planning on finishing the wood with oil polyurethane and would like to remove these blemishes, if possible, before applying it.

Dark spots on wood

I've tried waiting to see if it would dry out but haven't noticed a difference after 8 hours. I've also tried sanding with 100, 150, and 220 grit sandpaper but didn't notice a difference either. Is there anything I can do to remove the blemishes?

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    "would like to remove these blemishes, if possible, before applying [finish]" You really have no choice but to remove them since they will very likely become more visible after finish is applied, not less. You need to plane and/or scrape and/or sand until the staining is gone. Out of curiosity why did you use T&G and not just joint the edges and glue those? – Graphus Mar 9 at 7:06
  • @Graphus I had seen someone use T&G for a desktop with the same wood and thickness in the past, so I figured that was the way to go. It is only 0.75 inches thick. The boards are still glued together at all joining points. – sawprogramming Mar 9 at 21:00
  • OK for future reference if you joint properly — surfaces are smooth and straight, square to the face or complementary angles (e.g. 89° and 91°) — and you clamp up well (PVA and foaming polyurethane both need high clamp pressure for max strength) then the joins between boards will end up stronger than the wood around them, which of course is as strong enough :-) You can of course choose to use T&G anyway, but just bear in mind that you're doing it as an alignment aid, not for strength. It's a lot of effort to go to to only help with alignment, plus you lose a small amount of width too. – Graphus Mar 10 at 9:56
  • The above applies to all wood and all board thicknesses by the way, so rest assured you can rely on it for anything from pine to purpleheart. – Graphus Mar 10 at 9:58
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I followed @Graphus advice and continued to sand the wood using 80, 150, and 220 grit sandpaper. Thankfully, the spots were almost entirely removed:

enter image description here

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    The photo is a little overexposed but that looks so much better! Well done. – Graphus Mar 10 at 9:43
  • Excellent! Give yourself a check mark!! – FreeMan Mar 15 at 17:19

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