As a baot builder I like repairs to be done in the original material. Using epoxy over vinylester is not always good, and we always remove all traces of it if a job comes into our shop with epoxy over vinylester/ polyester. My reasons are:
the hardner in many epoxies is incompatable with polyester gelcoat and will stop it curing.
In my experience many epoxies are not weather proof and will simply peel off after a period of time. (west system is no problem!)
The mixing ratio on poly/vinyl is not critical and unless its way out the resin will usually go off no matter what.
Epoxies go yellow and look terrible.
Use of epoxy will usually cause problems finishing with gelcoats not sticking. (again west system has few problems)
Jim Boyer