On 11/04/2015 15:11, Dave Plowman (News) wrote:
In article ,
Fredxxx wrote:
Years of my father battling a crappy rustbucket Fiat when I was a kid
taught me none on the magic remedies work, with the possible exception
of phosphoric acid which almost seemed to do some good. Jenolite was
crap. Zinc preparations were useless.

I think Jenolite has moved on since your father's day. It now contains
your phosphoric acid.
http://jenolite.net/what.html

I think it always has done.

The snag with any of these preparations is they just treat the surface
rust. Just sand it down after it has dried to see what I mean. You'll get
back to rust. It might well work ok on fresh thin rust - but not on that
which has been there for some time. Hence the instructions telling you to
wire brush it. So with a spot on a car bodywork, you'd do more damage
attacking it with a wire brush than simply sanding down the effected area
to bare bright steel.

That argument would be true for any steel surface. The advantage of
using phosphoric acid is that it reacts with iron oxide to form a stable
and paintable surface. Removing more steel by sanding/grinding seems an
unnecessary way of reducing what good metal you ought to retain.