"John McCoy" wrote in message
. ..
"dadiOH" wrote in :

As Lew said,
Bondo is polyester resin filled with (mostly) talc. To my knowledge,
cured polyester doesn't "suck up water"

Actually, polyester is notorious for doing exactly that.
Google for "polyester blistering" and you'll find 1000s
of articles on the problem.

I've seen my share of blisters but the ones I have seen have been between
layers of laminate or separation of the laminate from a surface.

I DAGS and the best page I found is relative to fiberglass/polyester resin
on boats from the link below. I have copied and pasted some excerpts, CAPS
are added to draw attention.

All said and done, I wouldn't worry about using Bondo as a rotted/missing
wood repair


dadiOH
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http://www.hartoftmarinesurvey.com/guide.html
Further observation shows that bottom laminate constructed with the
commonly used orthophalic resins has a LIFE EXPECTANCY OF AROUND 30 TO 35
YEARS before deterioration due to blistering and resin damage has
structurally weakened the bottom laminate. This deterioration can often be
observed as flexing of the bottom laminate when hand pressure is applied
even on boats in the 40' to 50' range.

Fiberglass blistering is caused by one or more factors such as resin type,
contamination of materials, trapped gases, built-in voids, poor wetting out
of laminate, incorrect humidity or temperature and dry layup. Osmotic
fiberglass blistering is a process which depends on the temperature of, and
exposure time to, the water. Given the above mentioned factors, it is not
surprising that fiberglass blisters appear on a large number of vessels
which are KEPT AFLOAT FOR LONG PERIODS OF TIME IN RELATIVELY WARM WATER.

FIBERGLASS BLISTERS FORM ONLY WHEN WATER PENETRATES TO THE LAMINATE. THIS
WATER NOT ONLY DAMAGES THE LAMINATE BY FORMING BLISTERS CAUSING LOCALIZED
DELAMINATION BUT ALSO COMBINES WITH UNCURED WATER SOLUBLE AND HYDROSCOPIC
COMPONENTS IN THE RESIN FORMING AN ACID SOLUTION WHICH IS HIGHLY CORROSIVE
TO EVEN THE WELL CURED POLYMERS IN THE RESIN. As more water reaches the
laminate, more corrosive solution is formed and more resin broken down. The
effect is that of flushing the resin out from between the fiberglass
strands. A laminate so affected is often referred to as having been
hydrolyzed.