Gavin Broad wrote:
> Can I ask for Lister's exerience of dealing with the kind of rust that
> develops in the seams between spot-welded thin-skin panels please?
> Short of drilling out the spot weld and removing the panel for blasting,
> what is best?
Here's an approach that I used successfully on an early Ford Bronco. I had no
rust at all return for the six additional years that I owned it, despite
weathering our Utah winters, with the countless tons of road salt.
1) Scrape out the remains of the original seam sealant, if present.
2) Sandblast the seam thoroughly with a fine-grained medium.*
3) Vacuum out the remaining grit from the seam.
4) Liberally apply a thin self-etching zinc phosphate primer.**
5) Apply a second coat of the same primer, if desired.
6) Apply a coat of primer-sealer.
7) Apply a bead of urethane body panel sealant.***
8) Top coat with the desired paint.
One other tip: If you can reach the other side of the seam, it should be
treated in a similar manner. At a minimum, try to clean and seal the other
side to prevent moisture penetration from that side.
*For sandblasting, I used a "spot blaster" that has rubber tips to
concentrate the grit on a very small area. It recycles the grit into a small
cloth bag. This is an enormously handy device that leaves virtually no mess,
except when a bit of grit leaks past the rubber tip. Use a fine-grained
medium to penetrate as deeply into the seam as possible. Change the medium
periodically as it breaks down.
**Zinc phosphate primers are safer than zinc chromate and are about as
effective. A self-etching variant is highly recommended. This typically comes
with a separate bottle of alcohol and phosphoric acid that is mixed with the
primer base. It is fairly thin when mixed and should be applied liberally to
allow it to penetrate deeply into the seams before it has a chance to dry.
The phosphoric acid etches the surface of the metal and helps the zinc primer
adhere better and form a better chemical bond. The zinc compound has a slight
sacrificial anodic effect, and the pigments in the primer base help seal out
oxygen and moisture, although a primer-sealer coat is still recommended for
better sealing.
***You can get a tube of urethane body panel sealant from an automotive body
shop supply house. This can be applied with a standard caulking gun. It sets
up a little harder and stronger than silicone caulking, and remains flexible.
It also seems to adhere more tenaciously than silicone, and it is completely
paintable. Even if left unpainted, it seems to hold up well to direct
exposure to UV and ozone. More expensive than silicone, but excellent stuff!
Have fun,
Alan
This archive was generated by hypermail 2b29 : Sun Dec 03 2000 - 20:29:59 PST