Using Bondo to Fair a Boat Hull
1/5/26
This seems to be a controversial topic until it is explained.
Bondo is a generic name for automotive body filler. It is also
a brand name that is owned by 3M. From what I can tell
it appears that 3M has bought up most of the traditional "bondo"
filler manufactures in the US.
3M also markets their own name brand "bondo" type products. 3M
also makes marine "body" fillers to fair and fill fiberglass.
There seems to be three classes of marine fillers.
1. A polyester based filler with either micro balloons or
short chopped fiberglass strands
2. An epoxy based filler with "likely" micro balloons
3. A vinylester based filler with "likely" micro balloons
The cost varies from about $100 per gallon for polyester to more for
the epoxy, with the vinylester topping out near an eye watering
$400/gallon.
In the automotive end of the filler business there is really only
one type of filler and that is the polyester based fillers.
The difference is that some of them are "waterproof" and some of
them are not. This depends on what filler/stiffener is
mixed into the polyester resin.
The common "bondo" filler that has been sold for 5-+ years is not
waterproof. Meaning it can absorb water throughout the
material.
In traditional body work, the sheet metal was ground down with a
coarse sanding disk and bondo was applied directly to the shiny
steel. The bondo was leveled and sanded, then primer was
applied over the bondo filler.
The paint was responsible for keeping the bondo dry, otherwise the
bondo would get wet and the steel beneath the bondo would rust and
eventually the bondo would pop off the surface.
There is a waterproof version of bondo that is sold mostly be
3M. There is waterproof "Bondo", waterproof Dynatron, and
waterproof 3M brand fillers. I believe that all of them have
chopped fiberglass as the filler to the polyester resin.
All of these use the same 3M paste activator.
Anyway, if you look at the specs, the 3M polyester marine filler
with chopped strand fiberglass is pretty much the 3M autobody filler
with chopped strand fiberglass.
I can't see any difference.
So why use the marine version?
I have a 1974 Pearson 10M sailboat and some of the original fairing
and filler was beginning to loosen. I chipped and ground it
out in the spring of 2025.
I used am 80 grit orbital sander (a generic Fein design made in
China clone) to do the prep.
I made sure all of the surfaces were clean and sanded and free of
dust. I applied Dynatron (a 3M brand) mixed with the 3M
hardener paste using a squeegee (ala autobody style) and then let it
set. 20 minutes later it was ready to sand.
Leveled it as best I could and then applied bottom paint. All
in the same day.
How has it held up in Lake Erie water being submerged from June to
November? Great. Zero separation or cracking.
It works great.
3M, Dynatron, Dyna-glass is about $90/gallon on Amazon and other
locations.
Total Boat also sells a similar product but they use a liquid MEK
for activation. I prefer the paste myself.
The Total Boat product is slightly more expensive, but honestly its
fairly new.
Dynatron, now 3M has been making the chopped strand fiberglass
filler for at least 50 years. I made a repair with it in
the 70's on a Camaro and it has help up perfectly.
If you do autobody work, chances are you might have some Dynatron
sitting on a shelf. The product itself seems to have an
indefinite shelf life. But the paste activator goes bad
after a while.
You can buy the activator paste by itself. Its not hard
to find. 3M seems to make most of that now as
well.
Some marine places like Go2marine is "automotive" waterproof filler
for marine use. So the word is out.