poly is much nicer, flexes. Epoxy is stiff...
Are you talking about polyester ?
Gonna need to say something on that one :
Polyester should only be used in flat-bottom row-boats ; but
only in small ponds where there's no wind and you could walk
to shore if it breaks.
Polyester is what surfboard sharpers use, when there too Cheap
to lay down the money for Epoxy !!
The molecular weight-chains
that make up Epoxy are Much tighter than poly, but you can get
most any flex-ratio you want with epoxy , (from G-Flex to aircraft grade)
and everything in-between ; if your willing to do your
If you want your board to Flex, then build the blank Thinner ;
(stiffness increases by the Cube of the thickness).