How do I de-nib polyurethane after it dries but still maintain its finish?

Question

Sometimes when I apply polyurethane, for whatever reason, it's not completely smooth. Maybe a brush mark here and there, or a couple little nibs, etc.

The advice I see often is to sand the area lightly with a high grit sand paper.

When I do this, even when I sand very lightly, like one low pressure pass with 400 or 600 or even 1000, the texture comes out nice and smooth but the polyurethane is always scuffed with little white marks.

How do I avoid or remove the scuffing? How do I actually smooth nibs and other small irregularities out of polyurethane? The frequent advice to sand lightly is either incorrect, incomplete, or I'm missing something. I have never had success.

Also does the technique vary for gloss vs matte/satin?

Answer 1

How do I avoid or remove the scuffing?

Basically you can't. Until you get to really high grits (way above P1200) you will see the scratches and these are not suitable for de-nibbing.

How do I smooth polyurethane after it dries but still maintain its finish?

In general you don't just sand, you then polish the surface. Wet-sanding will reduce the scratch pattern somewhat as well, compared to sanding at the same grit dry, but the main 'secret' is that you then use some polishing agents to buff/polish the surface.

Traditionally the polishing of a dry finish was done with something like rottenstone. This is still in use today by some, particularly traditionalists, some high-end woodworkers and professional finishers/restorers.

You can in theory do this entirely with abrasive paper or film, but you have to go to extremely high grits and these can be quite costly. While that is how some do it it's really not necessary, today I think most people use something like an automotive finish polish e.g. Simichrome, Flitz or T-Cut to rub out the finish to its final high gloss.

How do I actually sand a nib out of polyurethane?

You can use the abrasives you're already using, but another tip you might like to try is to use the rough side of brown paper. This is just abrasive enough to take most nibs off without the risk of rubbing through the finish. It will leave the surface visibly less glossy, although if you buff hard you can get some polish back (essentially giving you something like a satin or semi-gloss finish with care).

The ideal situation might be considered to be where you don't need to sand any nibs or imperfections, and if you set up a dedicated finish room or zone where fluff or dust are kept to a minimum that's theoretically possible. But in practice this is something you will have to deal with sometimes so it's good to have a process ready to tackle it when it does occur.

Note: wiping on varnish can nearly eliminate problems with dust nibs (also brush marks, runs and drips). The downside is that because you're applying the varnish so much more thinly build is much slower and it can take many more coats (7+) to get to a fully-built finish.

Also does the technique vary for gloss vs matte/satin?

You're not supposed to work over the surface of reduced-sheen varnishes, anything from matt to satin or semi-gloss should ideally be applied and left alone.

At the pro or high-end level the goal with these is to get a smooth build using gloss and then apply one coat, two at the outside, of the reduced-sheen varnish as a final coat. Obviously you take great pains to try to ensure you don't have any dust in the environment before the final coat goes on.

A better way to achieve a sheen below fully glossy is to scuff the surface of gloss varnish. You can easily get anything from a subtle satin to full matt depending on the coarseness of the abrasive used (powdered pumice, fine grades of steel or bronze wool, Scotch-Brite or an equivalent).

Answer 2

When I do this, even when I sand very lightly, like one low pressure pass with 400 or 600 or even 1000, the texture comes out nice and smooth but the polyurethane is always scuffed with little white marks.

From memory, the directions I always see on cans of poly is to sand between coats, not to sand after a final coat. After sanding (fine grit sandpaper or 0000 steel wool), wipe off the dust with a rag soaked in mineral spirits, then apply the final coat in a dust-free environment so that you don't get any more nibs in the finish.

This website suggests the following technique to polish a gloss polyurethane finish:

You should use an automotive polishing compound and a buffer for big areas. Small areas will have to be polished by hand unless you have some funky buffing tools. Make sure the poly has cured for a minimum of 4-5 days at 70f. Do not try to polish an uncured film. Start gently, sand all the imperfections (wet 1000g) and then clean the surface well. Use a fine cut cleaner by McGuire's or 3M and follow the instructions on the bottle. Clean all residue when done with a damp cloth. Repeat the polish process with a swirl mark remover. Again, clean all residue with a damp cloth and buff clean with a cotton cloth.

Apply a final polish of your choice. This should give you a table that is like a mirror. Also, try this out on a test panel first to make sure you like the result.

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Also does the technique vary for gloss vs matte/satin?

I don't think you would want to polish a matte/satin finish since it would defeat the purpose to have it shiny. Also, matte/satin are much more forgiving in terms of surface imperfections, so that polishing regimen I pasted above would be overkill.

Answer 3

Before you try anything else, try "sanding" the surface with a piece of a brown paper bag. That is just abrasive enough to remove dust and smooth the surface without removing any significant amount of varnish, and is a lot easier than rottenstone or other more traditional polishes. Like any polishing/buffing process it will tend to eliminate tiny irregularities and make the surface shinier.