I took my rear deck out, glassed it all up, sanded it down, only to find out that while the fiberglass was curing, it bowed the deck to the point where it just wouldn't fit back in again;
It wasn't really noticable, but somehow the ****ed thing changed shape //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
So if you remove the dash and glass it, be sure that you have taken measures to help it retain it's shape.
If you glass it while it's still mounted (yikes) I guess it probably won't warp as badly. But good luck removing it after it's rock solid to sand it.
I think the mold-method is your best bet for a whole dash; if you're just talking about the trays, you may be able to remove > glass > sand > paint > replace without as much trouble.
Just giving you the heads up from my experience. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif