Here is an extract taken from the general FAQ page for System Three, a pretty well known epoxy manufacturer:
"Can I put polyester gelcoat over cured epoxy?
In general, polyester resins won’t cure properly or bond well to epoxy resin products without a “tie-coat” barrier resin in between. System Three SB-112 resin system can be used as a tie coat in between epoxy laminating or coating resins, and polyester laminating or gelcoat resins. Using this resin allows you to gelcoat an epoxy-built or epoxy-repaired boat."
Now, I have never had a failure of polyester gel coat to cure when applied over cured epoxy. But I have never tried to do so. All of the tales of woe that I have read about on the internet were second hand so I have no way of knowing whether the cure failure might have been due to other factors such as poor surface prep or failure to provide an adequate air barrier. I am also aware that individuals have successfully applied polyester gel coat over cured epoxy on many occasions.
I do know that if you frequent various boat building forums you will find the admonition against using polyester gel coat over epoxy frequently repeated. And I have also heard this same advice from a well-known individual who was intimately involved with composite canoe manufacture.
Is the notion that polyester resins won’t cure properly when applied to cured epoxy all a myth? Maybe so, but it seems to me that if well known manufacturers of polyester gel coat, well known manufacturers of epoxy, and experienced boat builders all advise against the practice, there is probably some basis somewhere for that advice. My guess is that certain conditions, involving certain epoxies and certain polyester gel coats may result in a failure to cure.