I just found this article hopefully it helps
Just a few facts that I know concerning the comparison of polyester
resins to epoxy:
Polyester is not an epoxy. It is a complete resin system that will
slowly polymerize when it's solvent is evaporated - though this may
take months. That is why MEKP is added. It is a catalyst - not a
component of the resin system - that speeds up polymerization, or
curing.
Epoxy is a true two-part resin system in that polymerization will
not occur - over any length of time - unless the two components are
mixed.
Why epoxy is a better barrier coat than polyester and why epoxy
doesn't blister...
Epoxy is 100% solids (high quality epoxy, anyway, such as WEST,
System 3, and others) meaning that there are no solvents - the lack
of solvents is why epoxy resin and hardener do not have much of an
odor, though some odor.
The weight of cured epoxy will be the same as when it was liquid.
Polyester resins have allot of a solvent called styrene and this
styrene evaporates as the polyester cures - this is why polyester
resin smells really bad. This evaporation leads to porosity within
the cured res. The porosity is how water eventually permeates gel
coat and causes blisters. All plastics absorb some moisture,
chemically speaking, but polyester absorbs way, way more water than
epoxy. This is why epoxy is an excellent barrier coat.
Epoxy is an adhesive. Polyester, though it has adhesive qualities,
is not as good for bonding as epoxy...
Polyester is a laminating resin intended to be used for reinforcing
fiberglass, just as it is in our hulls. While polyester is still
in the partially cured or "green" state, another layer of polyester
added will have a "chemical" bond. When we repair our hull, the
polyester is of course fully cured and the bond that is achieved is
strictly a "mechanical" bond. This is why surface preparation,
regardless of what type of resin is used, is VERY important.
Sanding increases the roughness and surface area and creates a
better bond. Polyester will stick to cured polyester, but epoxy
will bond much better. NEVER use polyester over epoxy, though.
Polyester resins also have a bad reputation for eventually
delaminating when used to glass over wood, such as plywood. Boat
manufacturers will glass over plywood with polyester resins and
boat yards make allot of money repairing rotted transoms in power
boats. Epoxy will soak into wood (doesn't cure as fast as
polyester helps here) and, being more water proof than polyester,
will stay bonded indefinitely as long as all of the wood is
encapsulated in epoxy.
So, why don't boat manufacturers use epoxy instead of polyester?
Some do. Allot of custom boats are built using epoxy resin. The
issue is efficiency and cost. Epoxy, though higher technology,
costs 2 to 3 times as much as polyester resin. Both resins can be
used to mold fiberglass, but polyester is more controllable by the
amount of MEEK catalyst added so production time is reduced- get
the finished hull out of the mold and get another one started.
Epoxy molded fiberglass must also have a surface finish applied,
polyester resins are used to gel coat the mold and then laminate
and out pops a shiny finished hull! Epoxy also has a downside in
that an amine blush, which feels "waxy" on the cured epoxy, that
must be washed away with warm water before successive layers can be
laminated.
