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Old 20th October 2011, 03:02 PM   #1
MCPete is offline MCPete  United States
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Default remedy for an uneven glossy finish

As the final step in applying a glossy (piano) black finish, I applied three coats of Minwax (oil-based) polyurethane with a foam brush, lightly sanding between coats with 220 grit sandpaper.

Now the speaker boxes have a good gloss from a distance, but looking at a surface closely, the finish is uneven. That is, I can see some irregular spots that are more glossy than others.

Is there a technique that I could use now that would make the top coat of the glossy polyurethane more even? Primarily now I'm looking for a remedy rather than learning what I did wrong. What I thought might help is wet-sanding with 300-400 grit sandpaper.

If interested, I'll describe the entire finishing process that I used, but my main concern is making my finished product more acceptable. Many, many hours of work have gone into making these boxes (of my own design)!

Regards,
Pete

Last edited by MCPete; 20th October 2011 at 03:27 PM. Reason: left out important detail
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Old 21st October 2011, 11:11 PM   #2
MCPete is offline MCPete  United States
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Has anyone else experienced this problem of irregular glossiness when applying polyurethane? Would you say that it stems from using a foam brush as opposed to spraying it on? I should say that the previous finish before applying the polyurethane was flat black enamel from a spray can on sealed MDF. I didn't see any unevenness in the black enamel.

-Pete
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Old 22nd October 2011, 12:28 AM   #3
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300-400 grit is way too course to produce a shine. You might have to go up to 1000-2000. Final stage is to polish with some product also used for automobiles. In Europe that would be Comorant as a brand name, and if not available in the US, I am sure there will be a similar product.

Irregular glossiness may be caused by insufficient hardening of the PU coating. First PU dries, which may happen in an our or two, but it is still not good for fine sanding/polishing yet. Depending on the brand, it may take from 1-3 days to really harden out. You know if the coating is not yet fully hardened when it sort of gums up while sanding, and/or when residue sticks to the sand paper. The sanding dust should come off as dry as flour.

vac

Last edited by vacuphile; 22nd October 2011 at 12:33 AM.
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Old 22nd October 2011, 02:24 PM   #4
MCPete is offline MCPete  United States
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Thanks for the feedback vac,

Yes, I was thinking that 400 grit is too coarse. I tried rubbing a section down with a damp cloth and some abrasive sink cleaner that is in powder form (Ajax brand here in the US). Even the Ajax made the surface dull and didn't really make the coat of PU more even.

Actually the finish is glossy enough as is. The problem is that some spots are more glossy than other areas. It is as if the PU bunched up heavier in some areas than in others. It could also be that the MDF was not adequately sealed and is absorbing the PU (through the enamel?) in some areas more than others.

Today I'm going to try sanding with 400 grit and applying yet another coat of PU. If that doesn't work then I may have to think of something else.

-Pete

Last edited by MCPete; 22nd October 2011 at 02:25 PM. Reason: typo
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Old 22nd October 2011, 02:35 PM   #5
jacubus is offline jacubus  United States
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Vacuphile is dead on with his diagnosis.
I tried the same finish some years ago.
I think the poly/ lacquer combination is incompatable. Your spotty finish may be caused by the lacquer trying to gas out under the poly.
I wound up stripping it back and just using the lacquer followed by progressively finer sanding. The fiinal steps are wet sanding with 2000 grit followed by buffing compound then wax.
Be patient. Give the lacquer several days to fully cure before sanding/polishing.
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Old 22nd October 2011, 11:33 PM   #6
MCPete is offline MCPete  United States
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jacubus,

Thanks for the input, but I didn't use lacquer at all. First I sealed the MDF and then painted the boxes with Rustoleum flat black enamel in spray cans.

Today I sanded the boxes having three coats of polyurethane (PU) thoroughly dry with 400 grit sandpaper (3M brand, expensive but good). As vac said I should, the residue from the sanding with 400 grit was a fine white powder. Then I applied another coat of PU, and it turned out much better than before. The final coat is now much more even.

This is the first time that I've worked with PU, but these are my impressions. On the can of clear gloss PU, the instructions emphasize putting down thin coats of PU. However I found that if I put down only a very meager or thin coast of PU, then the scratches from using 320 grit sandpaper showed through. So following the manufacturer's directions to use 320 grit between coats, I tended to put a heavier coat of PU down to cover the scratches and I think that the heavier coat caused problems.

With 400 grit as opposed to using 320, I was able to apply a thinner coat of PU and the scratches didn't show through; not needing to apply a heavier coat of PU I believe gave me a better final coat.

Now they don't look anywhere near as good as the pair of NHT SB2 that I have, but they look acceptable.
-Pete
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Old 23rd October 2011, 12:16 AM   #7
MCPete is offline MCPete  United States
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Sorry, in the above post of mine, I should have said that the manufacturer recommended 220 grit sandpaper be used between coats, not 320.

-Pete
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Old 23rd October 2011, 01:29 AM   #8
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Quote:
Originally Posted by MCPete View Post
Is there a technique that I could use now that would make the top coat of the glossy polyurethane more even?
It's a bit late now but in future you may want to apply Sanding Sealer before the Gloss Polyurethane.

I've never tried on MDF but Sanding Sealer over plywood seals the stained surfaces nicely for me. After that, one coat of PU Gloss with a brush gives me an even coat.

For more information on finishing, click here.
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Old 23rd October 2011, 01:29 PM   #9
jacubus is offline jacubus  United States
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HI Pete,
My mistake in assuming lacquer. I was thinking piano.
Im glad it turned out acceptable for you and you didn't have to strip it back.
220 grit seems excessive for flattening a finish. I usually go with 400 between coats then 800 and upward towards the final rub out.
Black is a demanding color to work with. The good news is that as you gain experience you will be able to lay it up instinctively and naill it every time.
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Old 23rd October 2011, 04:33 PM   #10
MCPete is offline MCPete  United States
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Quote:
Originally Posted by Michael Chua View Post
It's a bit late now but in future you may want to apply Sanding Sealer before the Gloss Polyurethane.

I've never tried on MDF but Sanding Sealer over plywood seals the stained surfaces nicely for me. After that, one coat of PU Gloss with a brush gives me an even coat.

For more information on finishing, click here.
Thanks for the suggestion to use sanding sealer. What I used as a first coat was a combined stain & sealer. I'm very happy with the deep black color that I got, but perhaps the sealing aspect of that first coat was not adequate.

You are right, Jacubus, black IS very demanding.

Regards, Pete
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