Salvage Sanding Mistake on Finish Coat - Woodworking Talk - Woodworkers Forum
 
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post #1 of 7 Old 02-17-2016, 08:24 PM Thread Starter
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Salvage Sanding Mistake on Finish Coat

Hi, first time post here. But I want to try to see what options I might have to correct a table finish that was 95% done.

I had a table made using rift-cut oak. I did basic sanding, then scraped it with a wire brush to enhance the 'divits' in the grain. To this, I applied black water-based dye stain. The finish needed to be completely black, so to ensure all the divits were completely filled-in, so no bare wood was showing, I had to apply 3 coats.

Everything was looking great, so I started my top coats. I am spraying General Finishes High Performance Water-Based Polyurethane Top Coat (Flat). After first coat, did light sanding with sanding sponge. Then cleaned and did second coat, this one pretty heavy on the top since it was flat and it was pretty warm outside, so no chance of runs.

But after the second topcoat, I tried sanding lightly with 400 grit sandpaper instead of the sponge, though I went lightly over it with the sponge after. UNFORTUNATELY, once I cleaned the sanding dust I now see a bunch of tiny streaks of raw wood showing through! It doesn't appear to be the divits, but rather parts of the raised grain possibly?!? So it looks like my 'light sanding' went through the topcoat & stain in a few place, unfortunately.

- Is there anything less then completely stripping it and starting over I can do to cover these marks with stain so they are black again? I was planning on doing a final 3rd coat of topcoat.

- Can I apply the stain over the topcoat? Would it stick to the areas that are showing raw wood? Or would it just come off or create weird streaking/inconsistencies?

Thanks!!!

Last edited by jethrodesign; 02-17-2016 at 10:24 PM.
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post #2 of 7 Old 02-17-2016, 08:37 PM
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I think you need to post some pictures. I can't imagine sanding it down to bare wood with 400 paper. It sounds more like you didn't get the surface clean enough and coated over some contaminate. If this is the case the streaks are suspended in the finish and needs to be stripped to fix it. If you've taken the hide off of it on the corners where the streaks are tiny you can color it in and continue with your finish.

The next time you wire brush the wood to enhance the grain I would do that first and then do your fine sanding afterwards. The steel from the brush color the wood.
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post #3 of 7 Old 02-17-2016, 08:57 PM
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I agree with Steve, it's highly unlikely light sanding with 400 grit sandpaper would cut through two coats of polyurethane. If the poly was dry. If the poly was still wet, well the poly will roll-up on whatever is rubbed over it.
My question: Are you absolutely sure your top coat was dry?
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post #4 of 7 Old 02-17-2016, 09:55 PM Thread Starter
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Hi, thanks for the replies! I've attached the best image I could get at this time using my phone. A bit hard to see, though.

(it can also be viewed here)

It's really hard to tell if it's bare wood or sanding dust. Even after I looked at it under a loop, it was still tough to say. But it definitely might be sanding dust. After looking closely, it might be more in the grooves than on top, but not 100% so. It definitely does not come off with compressed air or wiping. And it wasn't there after the first top coat.

The poly top coat was most definitely dry. It had been put on almost 24 hours prior to the 2nd sanding, and it was over 80 degrees yesterday, so it was dry to touch within 30 min or so. And it had been put on 1.5 hours after the first coat (once lightly sanded), when it had been closer to 90 degrees out.

Wondering why it is showing so much on the top, but really nowhere else on the table?!? The only differences were that I sanded the top with the 400 sandpaper, and probably was a bit more aggressive than the rest of the table. And the poly on the top was laid on fairly heavy yesterday.

My carpenter is telling me that he thinks we can fix it by tinting the next top coat with black dye (he has some). He wants to do the whole thing so it would be uniform, I'm a bit nervous and would rather do just the top first to see what it does, since the rest of the table looks fine. I'm not convinced it will be enough, but he thinks it will.

He also tells me this wouldn't have happened if I would have used sanding sealer. I was under the impression that was optional and best for treating unfinished wood prior to top coating. Should I have used that? Remember that I specifically am going for the divits in the grain, as is fairly common for modern/contemporary furniture finishes.

Thanks for any help here!!!
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Last edited by jethrodesign; 02-17-2016 at 10:28 PM.
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post #5 of 7 Old 02-17-2016, 10:36 PM
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You might try a fine point sharpie and try coloring the streaks. The grain of oak is open especially since you went over it with a wire brush. Think of it as grooves in the wood. Now, when you sanded the finish the grooves got the dust from the finish in the grooves and you put a clear coating over the top sealing it in. The only way to eliminate it would be to strip the finish off and re-do it. If you could turn the clock back to before you put the second coat on you could have used compressed air or a wet towel to clean the debris from the sanding out of the grain.

If you try the sharpie don't try to darken it with one touch. Lightly stroke it gradually darkening it. If you get too much try rubbing it with your thumb immediately before it dries completely. If you just can't rub it off alcohol should strip all the ink off.
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post #6 of 7 Old 02-18-2016, 03:24 AM Thread Starter
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Thanks again for the reply! It does make sense that the sanding dust may have gotten stuck in the grooves between 1st and 2nd coats. I guess I must not have noticed it before sanding the 2nd coat, but as bad as it looks, that is very surprising. And still curious why it's only really on the top.

I did try getting rid of the dust with compressed air first, then wiping with damp shop towels. I even then went over it with blue painters tape to try to get any large stubborn pieces of dust. Dust was really tough on black. Guess it still wasn't enough.

- Any thoughts on spraying with black tinted poly over this? Thinking sharpie might b tough to do well, not sure.

- If I decide to strip & start over, how far do I have to go? Any tips so it doesn't happen again?

Thanks!
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post #7 of 7 Old 02-18-2016, 09:40 AM
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Quote:
Originally Posted by jethrodesign View Post
Thanks again for the reply! It does make sense that the sanding dust may have gotten stuck in the grooves between 1st and 2nd coats. I guess I must not have noticed it before sanding the 2nd coat, but as bad as it looks, that is very surprising. And still curious why it's only really on the top.

I did try getting rid of the dust with compressed air first, then wiping with damp shop towels. I even then went over it with blue painters tape to try to get any large stubborn pieces of dust. Dust was really tough on black. Guess it still wasn't enough.

- Any thoughts on spraying with black tinted poly over this? Thinking sharpie might b tough to do well, not sure.

- If I decide to strip & start over, how far do I have to go? Any tips so it doesn't happen again?

Thanks!
I may have been wrong the white in the grain and may still be removable. Try wiping the finish off with the oil stain you used to stain the table with and see what happens. Completely wipe off the excess but check to see if the white streaks change color at all. If it at least turns gray you might be able to go over it with a tooth brush and mineral spirits and then wipe off the excess.

If it comes down to it the sharpie isn't hard. If you get too little on you can add more and too much and you can clean it off. If it comes down to refinishing it, that would mean using paint and varnish remover and removing all the finish and part of the stain. Then sand and start over.
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