Many epoxies have a small time window for re-coating for a chemical bond, which can be a problem if you have life.

I suggest, my builders to use System Three epoxy which gives a 72 hour window to add coats with a chemical bond.

To re-coat past the chemical bonding window time, I'd scrub with water and a Green 3M Scotch Brite pad.

See - http://multimedia.3m.com/mws/media/347828O/scotch-brite-hand-pads.pdf

Then dry and sand lightly with 220 No-Fil Adalox sandpaper which clogs less with green epoxy, than any other paper I've used.

See - http://www.amazon.com/Norton-Abrasive-Backing-Aluminum-Waterproof/dp/B001HWL98M

Good Luck!

Rob