How to prep cast iron downpipes for painting

[Jimk]

They said " free of.....rust"...? You painted over it? How long ago? How is it?

[nothanks]

I have quite a lot of cast iron downpipes and hoppers that need painting. They date from the late 1930s and have had quite a few coats of paint on them. Some are lightly rusting and some of the sockets have cracks. Life's too short to strip all the paint so I plan to sand them enough to get the loose paint off, then to treat the rust with a primer and then to paint with either a glass paint or the same Sandtext magnolia Trade masonry paint that the roughcast walls are painted with. Any suggestions for suitable rust primer/converter?

[tabbypurr]

There's only one treatment you can put on rust then paint directly onto, phosphoric acid. It turns the rust into a stable black coating. Other options all require removal of all rust to work. Fastest way to strip rust off in situ is an angel grinder + wire wheel.

NT

[nothanks]

Angle grinder and flap wheel has worked well on the CI gutters, but the downpipes are trickier. I'd forgotten about phosphoric acid - distant memories of using lots of it (Jenolite?) on a very dilapidated Ford Anglia in the early 70s. Thanks.

[Roland Perry]

In message [e-mail]>, at 22:52:27 on Sat, 25 Apr 2020, [e-mail] remarked:

Kurust, from Halfords.

[Harry Bloomfield, Esq.]

Which is phosphoric acid, with an added wetting agent.

[Andrew]

These days all the previously useful preparations like Kurust have become water-based and are almost impossible to use on anything other than a nice horizontal surface with absolutely no traces of paint or oil.

Working up a ladder applying this stuff is almost impossible without getting it all over you.

[newshound]

+1, you can also use the short pointy ones in an ordinary drill for re-entrant features around brackets, etc.

[polygonum_on_google]

I have used this Zinsser paint for other things, including metal with rust visible (but not loose). For my purposes, it worked very well.

The company said (via Screwfix):

Hello Thank you for your recent enquiry. Zinsser AllCoat Exterior water based satin paint is suitable to be used directly onto the cast iron gutters (exterior of these only) and downpipes, with no primer required. Two coats of the AllCoat Exterior paint will be required to be applied by brush or roller at a coverage rate of approximately 10m2/litre, when the air and substrate temperature is 10oC and rising. The second coat of AllCoat may be applied after approximately 1 hour in normal drying conditons. We recommend to ensure the surface to be painted is clean, dry, sound and free from any surface contamination such as dirt, oils, grease, mould, mildew, rust, loose or flaking paint etc that are likely to affect the performance of any subsequent coats of paint; to ensure this we advise the following prior to painting, clean down with hot soapy water, rinse with warm water and when dry have a good wipe over with methylated spirits to promote good adhesion. I hope this helps.

[nothanks]

[polygonum_on_google]

I removed loose rust - used either wire brush or some abrasive paper. Ended up with some areas which were still rust-colour but not worth it (for me) to carry on until it was perfect.

Then I painted over it.

I think it was three years.

Last I saw, it looked OK.

The biggest item was a sign that was left outside in the day, brought in at night, but was exposed to lots of horrible rain and wind.

I used isopropanol to wipe it down rather than meths.

Perfect finish was not required.

I wish I had some metal guttering and had used it on that. As it is, I can only make a suggestion and report my findings for my use.

What was wonderful was the ease of brush cleaning. As easy as emulsion.