On 09/05/2021 13:29, AJH wrote:
On 09/05/2021 11:43, John Walliker wrote:
Phosphoric acid is good.

I've got some 80% strength what dilution? Why would it be better than
hydrochloric acid?

I am thinking of unearthing a set of bodywork books from the 1930s that
my wife's granddad gave me shortly before he died in the 60s. He was an
old school pane beater and before resin fillers they would beat out the
dents and use lead to get the final finish as a skim.

Phosphoric will convert the oxide (rust) to an adherent layer of iron
phosphate. Good if you are going to paint, less helpful if you wanted to
tin. Hydrochloric if you want to pre-treat (active fluxes are doing the
same thing).