SOME EPOXIE may have oil in it not advised, Used regular fiberglss cloth and rosin on inside transom skin and same or fiber glass body filler on outside to re-do where bolt holes had wallered out. Then redrilled and used motor mount holes + 2 other 1/4" holes I decided to drill for inside and out side pieces of plywood cut to support transom skins to prevent warpage when seacast is poured. Seacast heats up and swells when it hardens thats why both inner and outer transom skins were supported w/ plywood.
Also used tread release on all bolts a must for removeal and liked having threaded bolt holes for motor instalation.
I wouldn't want wood left in transom after seacast job especiall near motor mnt bolts. I was going to use some thin cut up fibergallass pieces from a camper topper for spacers but didn't need to.
Also used plenty car wax on ply surface and lots around bolts area where went thru ply. Made circles out of spongy weather stripping in uneven areas where bolts went thru. inside -circles 1/4 " larger than bolts dia. glued in w/ liquid nail --to stop leaks in uneven areas only on inside skins .
I was ok to use marine-tex epoxie based filler on transom cap & other places after seacast pour complete.

seacast bonded fine to the old epoxie left behind after rot wood removal. probably just wouldn't use new epoxie before pour . seacast has a good manual u can down load dealing w/ most issues and procedures.

also I could of made a spacer out of the fiber glass 2' x 1-1/2 " piece I cut out of the transom cap. just would of cut a rectangular piece out of old cap so it would fit a little tight when hammered down in transom space near bolt area