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.
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.
Do you think primer is necessary?
Paint with bitumen, sprinkle stone on
NT
You can buy anti-corrosion primers that need to be painted onto rusty metal and then only need a top coat. Much quicker and easier. As it is about 20 years since I used one on a corrugated iron roof that has not needed treatment again, the name of the stuff has slipped my mind. I bought it from an engineers' supplier, rather than a paint shop, but I expect that professional paint suppliers will have something similar.
Colin Bignell
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