: . . . I've washed the damn boat down so much there
: couldn't possibly be any trace of blush left. I rolled the epoxy coats on
: at the same time so no temp differentials between sides. . .

I'd opt for the not-yet-cured-enough-epoxy theory.

Why oneside and not the other? I'd guess the side that sands well is the last area you applied epoxy to. If so , the batch had a chance to get warm in the pot and start to polymerize before it was spread out. Once it is applied the polymerization reaction occurs over a fairly large area, and without it being in a concentrated area there is not a lot of heat generated to promote further polymerization.

You can speed up the process with a gentle application of heat. You only have to raise the temperature about 5 to 10 degrees C or 15 to 20 degrees F to really accelerate the curing.

Get a few 60 to 100 watt light bulbs and lamps to hold them. Place them 2 to 3 feet (a meter or less) from the boat. Let them stay on for 10 hours or more and they should bring the surface up to about body temperature--which is hot enough to set the epoxy, but not so hot that it gets messed up. Check after an hour to see that the surface is not too hot to touch. If it gets too hot, move the bulbs back a bit. If it isn't warm enough, move them closer.

Depending on the number of bulbs and lamps you have you might do an entire side of the boat overnight, and then do the other side. Or you might just do a quarter of the hull.

Once you get the epoxy hard enough to sand it will still take several weeks for the epoxy to finish the polymerization process, so expect the surface to get tougher as time goes on. Again, you can speed that process with heat. Some people build "heat treating ovens" out of corrugated cardboard and bake their epoxy for a day or more at temperatures near those of a hot cup of tea. But different brands of epoxy seem to like different temperatures and times for heat treating, so check with the manufacturer for advice on this.

Hope this helps

PGJ