What are the pros and cons of applying more coats of polyurethane? I know with more coats the flaws are amplified, but doesn't more coats protect the wood better?
What are the pros and cons of applying more coats of polyurethane?
There are really no cons to applying more coats of poly, within reason. Cost does go up with each layer of course but this is inherent to all finishing; and you do get something for the additional outlay, be it improved looks or surface protection.
With each additional layer there is the chance of additional surface defects — blebs, dust specs etc. — however, these can be almost entirely avoided with proper setup and application procedure (and can anyway be fixed relatively easily if they do occur).
Fact is today it is common to under-varnish, because very often the focus when finishing is on looks rather than protection. For example, if varnishing a piece with wipe-on poly it's not uncommon for woodworkers to use just enough coats to get the sheen they want, which may mean 3-5. But wipe-on poly is diluted polyurethane varnish, so each layer is inherently thinner than regular varnish and the wiping application also tends to deposit a thinner coat than brush-application would anyway.
So six coats of wipe-on may only be the equivalent of only two coats of regular poly. And three or four undiluted coats is often the ideal number to reach the full protective potential of polyurethane (primarily maximum water-resistance).
I know with more coats the flaws are amplified
That's not really the case. It can be, but there are many variables. And often you seek to, and can, rectify flaws with each additional layer, building towards the final blemish-free finished project.
I think that its an advantage to use multiple light passes instead of one or two thicker coats.Ben Blackmar in the February 5, 2014 edition of Fine Woodworking says that he uses two coats of wipe on to start, multiple coats applied with a brush and a final top coat wiped on. Make sure you use very fine sandpaper between coats to level the surface and that coats are completely dry before you apply the next one.