Actually there are 2 epoxy resign's that can be gellcoated over. Ringleader did it to the stem of his boat using WEST System. I saw the boat 3yrs latter and it's flawless. I have never done it.
The other epoxy resign is made by System 3.
http://www.systemthree.com/p_sb_112.asp
Recommended Uses:
It was especially formulated for use in building and repairing sail and surfboards over polystyrene cores. It can also be used for coating and laminating. SB-112 is unique in that polyester resins and gel coats may be bonded to it without the use of "tie-coats". We recommend that polyester be bonded onto freshly sanded SB-112 epoxy within 48 hours of cure.