In message , Theo Markettos
writes
Djornsk wrote:
It's a long time since I did that kind of thing but you would really be
better off with an angle grinder with flexible disc which would do the
initial preparation in minutes. AIUI the best material for reinforcing
and building up the surface for a long term repair is lead rather than
body filler, and the following link may be useful if you decide to go
down that route:

Interesting... I hadn't thought of that approach. Is 'tinning' the surface
the final treatment, or does it need further painting? I have one small
dent that needed filling and I've filled it with 'Isopon' epoxy filler - but
that was easily sandable down to bare metal as it only had light rust.

This surface is upside down, so anything that requires gravity to attach
isn't going to work - solder paste might stay on by surface tension, but
significant blobs of molten solder wouldn't.

Do you need a serious hot air source? I just bought a cheap hairdryer as I
couldn't see the difference between that and a cheapo hot air gun - but I
doubt either would be hot enough to metal solder. Gas torch?

Once you've got rust beneath the car, you are better off abandoning
attempts to remove it and painting over.

Except for the parts you can see (around the edges of the wheel arches
etc, which should be painted, if you can), you are better off cleaning
as much of the dirt and rust as you reasonably can (high pressure hose,
wire brush etc), and when it is thoroughly dry (use hot air gun or hair
dyer if necessary), spray with Waxoyl (or similar). Several thinned
coats are better than on thick one. In narrow nooks and crannies, spray
with WD40 first.

Don't worry too much about getting absolutely all the dirt off. Dried-on
dirt soaked and impregnated with Waxoyl is a pretty good anti-rust
protection.

In places where the coating will get a good blasting from road grit and
stones, you could finishing off with your own mixture of Waxoyl and
underseal. [This sticks better than trying to underseal over a Waxoyled
surface.]

Finally, don't forget to top up the Waxoyl spray every six months or so.
--
Ian