Several years ago, a repair guy took care of a small area of rotted wood on
an exterior window sill by gouging out the rotted area and filling with some
sort of bondo-like wood filler. It seems to have held up pretty well. Any
idea what the stuff is called and where it is available?
Thanks in advance . . .
It may have been.
Several package it marketed for wood w/ tan/brown catalyst instead of
the typical blue/green for automotive -- Minwax brand is one usually
readily available. (It doesn't indicate what it is on the label, but
when I last checked it shared MSDS data w/ the Bondo-products which is
pretty much a clue... :) ).
If you do use an automotive filler (much cheaper, generally), choose
one w/o the fiberglass as it may tend to "fluff" more and leave a
raised grain effect that's more effort to eliminate.
Most box stores have the MinWax kits. They consist of a penetrating
'hardener' which is simply a high solids lacquer, and ordinary Bondo.
For small jobs, it is pricey but effective and in my experience,
pretty durable. You could probably the get same results with an oil
base polyurethane varnish allowed to soak well into the wood and
allowed to cure, then built up with standard Bondo. Use whatever
filler hardener seems appropriate, red, white, or clear. Best
selection will be found at auto body supply stores, Farm and Fleet,
and similar outlets. Study up a bit on autobody filler tools and
techniques if you have any large fussy detailed areas to repair.
As everyone has mentioned......Bondo can be used successfully for wood
BUT I used a Bondo on exterior redwood sills ~20 years ago and it
FAILED in less than a year! I used Bondo instead of an epxoy based
wood filler (Bondo was about 1/3 or 1/4 the price of the epoxy
I called Bondo tech support & asked them about this.......the tech rep
said "we do not recommend Bondo for the repair of redwood in exterior
To which I replied "I guess I missed that warning on the label".
I switched to www.Abatron.com products; Liquid Wood & Wood Epox.
Expensive stuff but bullet proof. Needs to be painted to protect from
UV but I have some 20year+ WoodEpox that I shoved into holes in a
sttuco'd wall that get morning sun for about 2 hours. The stuff has
been unpainted & has held up very well.
The Abatron products that I have used have excellent
performance......I used then 20 years ago & again 2 years ago (to do
some addtional work, not re-work).
I wood use Bondo for interior work without a second thought BUT I
would never use it again on exterior work. YMMV.
The stuff is a lot more $'s than Bondo.
Bondo works just fine on cars, even those that are used outdoors >G<
Epoxy resins are simply more $$ than polyester resins. The
characteristic odor of uncured Bondo is from styrene, a reactive
diluent which makes the product even less $$. So buy whichever system
does what you really need.
Yes, Joe...Bondo works fine on cars......even those used outdoors,
just not cars made of redwood (what other woods?)
According the Bondo tech support, Bondo (at least the formulation sold
20 years ago) is not for use on redwood in exterior apps..... I wonder
if there are other species of wood for which Bondo is also "not
The take away message from Bondo tech support was "Bondo is not good
for exterior use with redwood".
I extrapolated it to.....I don't risk using Bondo for exterior wood
Having experienced an exterior repair (of redwood) that failed, I
decided to not take that chance again......
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
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
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 does not last long on cars if used to repair rust holes. No
matter how well the rust is removed to bare metal, bondo HOLDS
MOISTURE. While the surface is painted, the rear of the bondo in the
hole is exposed, and absorbs water. Thus, it rusts away the metal
around it. I'd suspect that used on wood, it must be well sealed from
moisture or it will cause rot.
I used some JB Weld on a rotting spot in my wooden garage door. I dug
out the rot, and poured the JB weld in the hole. I applied another
coat later to form the surface. It's been a permanent and durable
patch, and I know it does not absorb water.
That i sexactly what happens. The bondo lasts maybe a year or two and
then falls out. You need an epoxy filler not a polyester filler like
One such product that comes to mind is www.rotdoctor.com. Usually you
scrap out the rot and then use a watery "stabilizing" epoxy to
penetrate deeply and solidify any rotted wood fibers. Then you use the
putty like epoxy to fill the cavity.
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