My sister-in law was recently repairing the finish on some doors and door moldings and ran into a problem neither of us understands.
Quoting from her email:
"[after sanding to bare wood] I then varnished it with Minwax Wood Finish that Penetrates, Stains, and Seals. I waited for about half a year. I just coated it with Minwax Polyurethane Clear Semi-gloss. While it was drying, white patches developed under it or within it. They weren't there before. It actually looked better with just the wood varnish on it, no gloss."
Presumeably to fix this the surface will need to be scraped, stripped or sanded back to bare wood and finished again, but how can this problem be prevented second time around?