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I'm working on a project that will have wooden inserts sliding into a groove. I've done something similar before and ended up with too small of a groove after a few coats of paint (even though I allowed something like a 1/16" clearance) and ended up sanding down the inserts and repainting.

So my question comes down to: How thick is a standard coat of oil based polyurethane going to turn out?

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One side? Two sides? Two coats each? Four each? There is likely to be a fair amount of variability depending on the project. –  bib Aug 3 at 0:39
    
One coat, one side. I guess I should have worded that more clearly. The rest can be extrapolated. Although the last coat may add a small amount extra because you don't sand it. –  Doresoom Aug 3 at 0:47
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depending on use, it may be inadvisable to finish the sliding surfaces. Surfaces that slide against each other frequently are usually left unfinished, and perhaps lubricated with wax - like wooden drawer slides, etc. –  aaron Aug 4 at 16:10

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Finish is very different from paint in this regard, paint (particularly latex) builds much heavier and never really cures as hard as a good finish so it always feels grabby on sliding surfaces. The first coat of any finish doesn't really add any thickness as it mostly soaks into the would fiber. And unless you plan on adding some very heavy or numerous subsequent coats it's not normally necessary to allow extra clearance for your finish. If you need to put a number to it, a 1/32 of an inch should be more than enough to accommodate any finish. If your finish is heavier than that you're doing something wrong.

If you have problems with sticking after finishing it's likely a result of the pieces not being fully cured (which will make them act sticky) or not fully sealing your mating pieces which allows the fibers to swell and shrink undesirably. Seal them well but let them really cure out before inserting them in their grooves. You should also apply a layer of bee's wax to the mating surfaces to help reduce friction.

Really the job of creating smooth operating sliding components is done during the building process. Wood movement must be properly accounted for both in the construction and even the selection of stock. Mating parts should be of the same species (ideally from of the same board) to ensure they expand/contract at the same rate. Also, a piston fit drawer, for example, can take hours of meticulous fitting to operate smoothly so take your time. Nothing you do or don't do during the finishing process will make up for skimping on these processes, so spend the time getting the fit and construction right before you head to the finishing room. Good Luck!

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Last time I measured it was right around 1/64th of an inch but I guess you could round it up. –  DMoore Aug 12 at 18:34

Wet and dry surface coating film thickness calculations are complicated and dependent on several factors (varnish/paint solids content, applicator, application method), an "average" per coat thickness, if one was pressed to give it, would be 35 microns dry thickness for alkyd varnish. Paints vary widely.

Why do you not paint or varnish after assembly?

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I didn't varnish after assembly because they parts aren't permanently installed in the groove. –  Doresoom Aug 12 at 16:05

We had a conference room table at my work with a polyurethane finish on it that was flaking off. Since I am a nerd, I measured the thickness of one of these flakes: .003 inches. FWIW.

I also measured the thickness of a few other things to compare this to: a thin planer shaving was .001". A noticeably thick planer shaving: .01"

In a recent blog post at Bridge City Toolworks, I read that the difference between a tight-fitting joint and a loose-fitting one is .002 - .003. which, of course, depends on the situation.

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