On Jan 15, 5:57*pm, BobK207 wrote:

snip

So the only different between epoxy resin and polyester resin is cost?

No. The characteristics of the two systems are quite different.
Speaking in generalities, epoxies are very low cure shrinkage, very
good adhesiveness. Cure can be effected by numerous types of
coreactants and catalysts. Response to fillers varies all over the
place, typically 50% for a mineral filler to 90+% for zinc powder.
This gives competent formulators like West and Abatron a huge palate
of materials to achieve the desired properties. Consider this: the new
Boeing Dreamliner is said to be all carbon fiber/epoxy.
Polyesters cure by a free radical mechanism initiated usually by
organic peroxides. Cure rates are often much faster than epoxies. From
a resin standpoint, many of the same fillers will work with either
system. The poyester resins are often better at UV resistance than
epoxies, but the latter can do quite well if hardeners are chosen
carefully. Rubber compounds cure by the same mechanisms as polyesters
(as do vinyl resins) but have few inter chain cross links which makes
them elastomeric.
Bottom line here may be Bondo is good outdoors on any substrate
properly prepared. Likely the world renowned Forest Products
Laboratory mavens in Madison, WI would know why your redwood to Bondo
interface failed.
Cheers,

Joe