On 10/03/2013 18:45, Gib Bogle wrote:
I want to check that I have the right procedure for dealing with
superficial rust on my galvanized steel roof. I understand that the
sequence is sand the rust off, wash with water, apply rust killer
(phosphoric acid), prime, paint. A couple of questions: Is it possible
to use the rust killer without sanding? The sanding step is the
hardest. Is primer necessary, or can I use another layer of the top coast?

Is this traditional corrugated iron, or the sort with stepped flats, or
completely flat? Unless completely flat, sanding isn't particularly
easy, I would tend to go with wire brushing with a steel brush (either
manual, or disk or cup type in a power tool). I would certainly remove
flakey and superficial rust, personally, but not worry so much about
thin or well adhered stuff.

Lots of google links, e.g. to Dulux Trade Mordant solution.

This one gives you more than you would ever want.

http://www.galvanizeit.org/about-hot...vanized-steel/

I have seen arguments that it shouldn't be painted at all.