On Jan 15, 5:25*pm, Joe wrote:
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 polyester 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


Thanks for the details, I was a little worried that you were going to
say that polyesters could completely replace epxoies...........

My recollection as to the Bondo tech support explanation for the
Bondo / redwood failure.....the naturally occurring chemicals in the
redwood prevented good adhesion and proper curing. Painted over with
oil base primer & paint the Bondo cracked and failed. Hence my
reluctance to use it again outdoors. Probably unjustified
superstition but I never wanted to have to re-do an exterior wood
restoration where Bondo failed again.

I've had great results with epoxies & I know the epoxy products will
easily last decades.

Actually I had my chance to ask the FPL guys about it at a wood
conference in New Orleans ~94 but I was too busy quizing them about
water repellents / wood preservation and completely forgot about the
Bondo disaster.


cheers
Bob