Polyurethane Book Case is Rough, Sand in Place?

Question

There is a book case in my parents' house that me and my father built about 6 years ago. It was built to fit around a dresser and mirror. It's a log house so everything is "rustic". We had to build it in three pieces to fit it in the room it's in. It's not the best book case in the world. We just needed something and didn't have a lot of nice tools setup at the time. I'd do plenty of things different if I did it again.

The problem is the finish. We did one, maybe two coats, of polyurethane. I can't tell you the brand or whether it was water or oil based. We just brushed it on, being careful not to have drips run down the finish. We installed it and it has housed my books for almost a decade.

The issue with the finish is that it feels like the wood grain is slightly rough. I don't notice it so much where the books sit as with the top and sides. When I used to dust it, it really caught the duster. Now, my books are still there and it bothers them. I can remove my books temporarily, but I don't care enough to unscrew the L-brackets holding it to the wall and I really don't want to carry it downstairs.

Could I use a high grit or wet sand paper to knock the finish down to where it's smooth enough not to catch the duster? Also, would there be a lot of sanding marks. Minor marks that might stand out on a highly polished table wouldn't matter here. It's just made out of some home depot boards and has a generic polyurethane on it. The rest of the house is wood/rustic. I don't want to put a lot of highly visible blemishes on it, though.

They actually have a similar problem in one of the bathrooms. The interior walls are tongue-and-groove and they'll catch towels and washcloths if you try to wipe them down. You have to wipe them down, because they also catch dust. I used to just vacuum the walls.

Answer 1

Could I use a high grit or wet sand paper to knock the finish down where it's smooth enough not to catch the duster?

Yes sand it, but no need to wet-sand or use a particularly high grit. If you were 'flatting off' and buffing/polishing the surface you'd want to do that (both wet-sand and go up to a high grit) but you can't in this case with the piece in situ and because there's not enough poly on the surface.

Also, would there be a lot of sanding marks.

Yes there will be visible sanding scratches unless you use a very fine paper which isn't good for smoothing off rough finish. It's not just that it's inefficient (you should try to always use the coarsest paper that'll do what is needed) it'll tend to clog and you'll burn through paper fairly quickly.

The ideal fix for this in conventional finishing would be:

• sand lightly following the grain
• dust off
• apply one or two light coat of fresh oil-based poly (thin varnish to wiping consistency, wipe or brush on)

If you don't want to go to this trouble for the bookcase that's understandable, but I'd definitely do this for the tongue-and-groove in your bathroom! I've been in a house with rough wood like that on the walls, it's not pleasant when you accidentally brush against it :-)

A variation on this would be:

• sand lightly in the direction of the grain
• dust off
• apply a coat of paste wax and buff off after it's dry

For either of the above, use med-fine paper. That's anywhere between 240 and 320 grit and sand lightly — you're just looking to knock off the high spots, not trying to sand the surface uniformly (if you did attempt to do this you'd surely break through to bare wood in spots).

According to Bob Flexner there's no need to back your paper with a block for this type of sanding, so just folding over the sandpaper and holding it in your hand is fine since you're just skimming over the surface.

More reading from previous Answers if you're interested:
How do I de-nib polyurethane after it dries but still maintain its finish?
Polyurethane or similar on multiple sides at once, quickly, no drips

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An alternative approach you might want to consider is scuffing the entire thing. This way you'll get a uniform scratched surface where the scratches from the initial sanding won't stand out.

So in this case you'd:

• sand lightly in the direction of the grain
• dust off
• go over every surface with a nylon scrubbing pad, grey Scotch-Brite or steel wool (00 grade is probably fine enough, but use 000 if you want a finer surface).
• dust off

When scuffing poly like this you don't need to press hard or work the surface for very long, just a few passes using the weight of your arm is usually sufficient.

Note: this general method is great for converting any high-gloss finish to a semi-gloss, satin, semi-matt or matt finish, depending on how coarse the scratch pattern is.

Answer 2

We did one, maybe two coats of polyurethane.

Depending on how thick your coats are that might not be a lot of finish. Sanding might be an issue as you could easily sand down to wood after only one or two passes. I would venture carefully here if you are trying to avoid having to refinish. Honestly you might have to anyway.

Instead of sanding you could also consider card scraping. This would make for easier clean up. However depending on how rough the surface is it might not be the best tool. If you do need to sand consider a finer grit and try testing first where people are less likely to look. Once you remove the material you desire you might need to add another coat and/or buff the finish.

That roughness you speak of is likely the grain raising from the application of poly. A very good related answer from Graphus would be worth a read here.

Answer 3

Yes go ahead and sand it. When I do my show tables for selling at rabbit shows the polyurethane does the exact thing you are talking about. I take 150 and do a little bit, because it does not take much to start smoothing the polyurethane out. Then I cork up from there, for me 220, 400, 600. 220 works very well I just love the really high grit. And for 'is there noticeable sand marks' I would have to say no. When I do mine you can not see the sanding marks. But I am using Satin so that may make the difference. I do not know about semi-gloss or high-gloss.