Wipe it down with acetone, use epoxy resin. Polyester resin doesn't stick well to anything. Even itself. It only bonds together when the previous layer hasn't fully cured. I use West Systems. They have several additives that can increase strength and adhesion.
I agree, it's really hard to beat the West system for repairs. Also can make a putty with it and micro-balloons or cavesil and pack it into joints or use as a filler.
That's Cab-o-sil: http://www.cabotcorp.com/solutions/products-plus/fumed-metal-oxides I spent a summer building fiberglass boats in less-than-ideal working conditions we joked that the fumes were melting our brains after one guy had trouble saying the name w/out stuttering, and we all started saying "ca-ca-ca-ca-cabosil."