I apologize for the length of the following. But I think the material
will be useful.
Scott Paisley asks how best to apply bondo. I have applied a lot of it
over the years, much of which has stayed on. I think the principal cause
of failure is that polyester body fillers are not waterproof--in fact they
seem to absorb water. So any "repair" with bondo that does not
encapsulate the bondo and seal all water entry points will fail. The
"classical" method of patching holes in rocker panels is to stuff
newspaper in the rocker to back up the bondo, and then to trowel the stuff
on. That is a perfect example of how NOT to do it.
My suggestions:
1) Surface preparation:
Any holes must be repaired so the bondo will be applied to solid
continuous metal. Continuously welded seams are best. If you must rivet
a patch on for economic reasons, seal the seams somehow (e.g. with seam
sealer or RTV) so water cannot get in. The metal should be bumped out as
close to the original contour as possible, but should at no point be above
the original surface.
I consider clean, unpainted, unrusted metal to be the substrate of choice
(freshly sandblasted is good, or after grinding with a 36 grit sander).
However, Ditzler (Pittsburgh Paint & Glass) tout their DP40 and DP50
catalyzed primer as being an even better surface for bondo than clean
metal. DP40 can be painted over without sanding if the topcoat is applied
within a short time (I think 48 hrs). I assume it is best to apply the
bondo within the no-sand interval. I have tried this, and will know in a
few years how it worked. If it works, it is nice, because one can
sandblast, epoxy prime, and then have a day or two to start applying
bondo. Warning: not all catalyzed primers are the same, I find. Bondo
applied over Dupont etch-type (Variprime?) primer peels right off :-(.
And a non-chip coating I wanted to use lifts the Dupont primer. (Guess
how much I like the Dupont primer.)
2) Choice of "bondo." I have used three types of polyester fillers. A
type that is strengthened with fiberglass strands, conventional bondo, and
a very fine grade intended for final finishing. The fiberglass reinforced
is nice where one is worried that flexing of the surface may eventually
loosen the bondo--the fiberglass reinforcement lends strength. Otherwise,
I haven't found much use for it. It presupposes a thick coat; the
fiberglass strands make a good finish difficult, so one generally needs to
apply one of the finer grades for a top coat. I have had no trouble with
the final finish on conventional bondo, but the finer texture of the
finishing stuff can be nice for the last coat.
3) Mixing. Follow label instructions. One can fudge and increase the
hardener amount slightly in cool weather to hasten curing, and decrease it
slightly in warm weather to perserve working time, but use bondo within
the ambient temperature limits the manufacturer recommends. Mix very
thoroughly, but work quickly to preserve application time. Make small
batches, and clean the mixing surface thoroughly between batches to avoid
contaminating the new batch with chunks of the previous one. Plastic
pallets sold for bondo mixing are nice; at the rubbery stage the bondo can
be flexed off. Remaining material can be removed with lacquer thinner.
When the surface of the pallet gets hard to clean, break out a new one.
4) Application. Apply with a flexible applicator sold for the purpose.
The applicator should be CLEAN; any remaining hardened material from
previous batches will make gouges in the new material. Apply material as
soon as it is mixed, and stop as soon as (ideally just before) the bondo
quits being fluid. It won't stick if it has begun to turn rubbery. Build
up to the desired contour in thin layers, alternating application and
trial shaping (see below). Try to do as much of the shaping work as
possible by the way you apply the fresh bondo--I have made templates
similar to those plasterers use on decorative plaster work to form bondo
close to the desired contour. I used a template, for example, to form the
concave groove on a repaired portion of the license plate recess on my
bugeye. That can save a hell of a lot of difficult sanding later. Before
each new coat is applied, check the previous coat. Inadequate mixing may
cause spots to remain uncured, and you don't want to put new coats over
uncured material. If material does not cure, clean it off and start over.
5) (S)working. As it hardens, bondo goes through an early gel stage, to a
rubbery consistency, then becomes firm, finally hard. Caught just before
it turns hard, the bondo won't stick to tools, and can be worked
beautifully with a surform file, a body file, or even an exacto knife.
That is the time to work down ridges left by the edge of the applicator or
any other obvious high spots. After the material hardens, it may be
worked by sanding or by filing with a wood file or body file. Alternate
between application and working. As one approaches the final contour,
most of the work must be done by sanding, and more and more of the
material applied gets removed to leave only very small additions. On most
jobs, I think 3/4 of the stuff I apply ends up on the floor. For the last
coat or two, one may go to the really fine finishing stuff to fill
pinholes and minor imperfections. Wear a good dust mask or respirator for
the sanding, especially if any of the material applied contained
fiberglass. Sanding should be done dry, so there will be a lot of dust.
IMHO, ONE SHOULD NEVER, EVER WET SAND BONDO. The stuff absorbs water, and
then you have trouble.
6) Paint. I try to finish the work on a section, then apply a coat of
primer before I quit for the day to seal the surface and prevent moisture
penetration. DP 40 is good if the section is finished completely.
Lacquer type primer is OK if you are going to work the surface more soon
(thus removing the primer; lacquer type primers are not good water
barriers in the long term). Before final painting, I think the DP40 is
particularly important to help seal the surface. If you intend to wet
sand the body before final painting, put a contrasting color primer over
the DP40, and quit wet sanding when the DP40 is uncovered. That way, you
won't wet sand through to the bare bondo.
7) Miscellaneous. I can't confirm that bondo ever contained asbestos,
though I wouldn't be surprised if it did, but a lot of it did (and still
does) contain fiberglass. So I would be cautious grinding off old
material of unknown composition. Paint remover will soften bondo, as
reported on the list, and with patience remove it. And I have, as someone
also suggested, successfully removed bondo with a propane torch. It
seemed to me that it worked by softening the bondo, rather than by
expanding it and the metal differently, so metal distortion is not too
much of a worry (as always, YMMV). Do it outside, though, it stinks and
can burn, so doing it in the basement will not thrill one's significant
other. I know whereof I speak.
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