View Single Post
Old 14th Sep 2020, 11:33 am   #46
Michael.N.
Pentode
 
Join Date: Apr 2011
Location: Bradford, West Yorkshire, UK.
Posts: 174
Default Re: What causes this in polyurethane varnish?

Personally I'm not a great fan of really flat high gloss finishes, the type you can use for observing Neptune. I once varnished a guitar and did the usual steps of going through all the grits , polishing compounds and buffing. When I eventually finished I stood back and so admired the brilliant gloss finish I had produced. The very next day I looked at it and decided it had to be all rubbed back. It was just too plastic wood looking. The more I looked at it the more I hated the effect. There's no doubt that it results in great optical clarity but it's just too cold looking for my taste. These days I tone it down a little with the French polishing cloth, deliberately putting in very straight 'micro lines' to lessen the gloss a little. I would still call it a glossy finish (most people would). It's just not glass hard reflective as some.
That's just my personal taste though. There isn't a right or wrong about this kind of thing so do as you please. Most finishes can be taken to a pretty high reflective gloss. Usually the harder finishes a touch more.
As for the curing time you have to learn to judge that. With the solvent finishes you can literally put two coats on every hour or so. Alternately you can wait months between any two coats. The new layer will solve into the underlying coating. With finishes that cure through polymerisation it can be tricky. If you leave it too long you can get two very distinctive layers. When you rub back it can produce witness lines, rather like contour lines on a map. I'm not sure if that applies to the modern polyurethane finishes. My real experience is with old fashioned linseed/tung oil/resin based varnishes. Many of the modern finishes are cold solved, low VOC - very different beasts.

Last edited by Michael.N.; 14th Sep 2020 at 11:42 am.
Michael.N. is offline