If its not a temp. problem with regard to cure time, you might want to remove the soft epoxy. Dont even think about adding even a teeny bit of hardner or resin [not that you have thought to]. You may want to consider using MEK [methyl ethyl ketone] or vinegar to remove the epoxy. i dont kno which works better - never tried vinegar. But I can say MEK is a slow remover. Youll know you have removed all the soft stuff when it no longer clogs your sandpaper.

Pete