Dear Leew,when you apply a surface coat like this,the resin does not
have enough volume in order to "generate" a high exotherm.
This is why your resin inside the pot has been polymerasized.
My suggestions are:
1) Use a heating lamp
2) Apply a "mist" coat of PVA on top of your resin,in order to "seal"
the resin.(after you must sand it and clear coat your part)
3) Try to use vinyl ester .