Joe Bleau wrote:
I rolled out a 6' x 4' panel with waxed gelcoat. It failed to harden
in spots.

The gelcoat went on a plywood panel which had been prepared with mat
and 8 oz. resin. The resin hardened and was leveled through sanding
through 220.

The waxed gelcoat was prepared with a slight addition of a brown
coloring agent and was catalyzed at 9 cc per qt. It was applied with
a new roller. Most of it hardened beautifully but there are spots
which failed to harden.

QUESTION: Does anyone know what might have gone wrong? Could the
unhardened spots be hardened by spraying on a film of MEKP

DO NOT spray MEKP on the surface. It's a really bad idea.
or could they be coated with PVA or wax?

It won't hurt to try and if the issue is exposure to the air, it should
help. If the issue is something like not mixing well enough, the coating
won't help. Regardless of the cause, spraying a coating on the surface
and applying heat should result in a full cure.