On 11/03/2013 8:03 a.m., newshound wrote:
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.


Thanks! The roof is neither corrugated nor flat. It is something they
called Decramastic tiling. Each piece is about 1m x 0.5m, formed to
look like tiles, i.e. with concavities on the 'tile' faces. It was
coated with tar, in which fine gravel was embedded. This roof is old,
and most of the gravel has gone, and over much of it the tar has gone
too. The final layer of zinc protection has gone in many small spots,
where rust is developing. It needs replacing or full painting (there
are firms that do this), but I propose a holding action that will give
another couple of years before anything major is needed.

A steel brush on a power tool is an appealing idea. I'll check out the
Dulux product.