: so the trick question is what do you do when you varnish, captains
: flagship, your hatch cover and wait 4 days and it looks GREAT,
: and then buff with 400 grit for the second coat only to find out
: it NEVER cured and you can scrape it off with you fingernail.

Dan-
How thick was the film you applied?
Even with a thinned coat of spar varnish, 4 days would probably not be enough for a hard cure, in my experience.
Spar varnish is designed/formulated to remain flexible, and once the 'skin' has formed, it takes quite a while for the rest of the film to cure (oxidize??).

Also, UV exposure and air circulation can also speed up the cure a bit.

So, thin coats and lots of time/heat/UV/circulation are the keys, IMO.

BTW, I learned this 'the hard way', by applying nice thick syrupy varnish coatings on a project, trying to get that 'inch of plastic' look. What a mess!