It’s common to hear the instruction that it’s better to apply several thin coats than one thick one. Why is this so? Or is it?
What’s involved is drying time, nothing more. Thinner coats of all finishes dry faster than thicker coats. The difference is great enough that you can build the same thickness with several thin coats in less time than you can get that thickness with a thick coat. But the thick coat will eventually dry just as hard and perform just as well as many thin coats.
Shellac and lacquer dry entirely by solvent evaporation. The solvent evaporates much quicker from a thinly applied coat than from a thick coat because it takes much more time for the solvent at the bottom of the thick coat to work its way up through the finish and out into the air.
Varnish, including polyurethane varnish, dries by the crosslinking of the molecules in the presence of oxygen. It takes much longer for oxygen to work its way to the bottom of a thickly applied varnish than a thin varnish coat.
Water-based finish dries by both solvent evaporation and some crosslinking, so the result is the same. It takes much more time for a thick coat to dry than several thin coats.
Of course, oil finishes don’t dry hard at all, so coats must always be as thin as possible, accomplished by wiping off all the excess.