Fixing rust on painted corrugated iron roof?

Forums › Home Workshop DIY › Fixing rust on painted corrugated iron roof?

mb82

223 posts

Master Geek

#311269 1-Jan-2024 11:30

I have a corrugated iron roof which has been painted at some stage. It is rusting so I need to do something before it gets worse. Any tips / product recommendations on the best way to fix it (without replacing it).

Looking for advice to avoid having it rust through the paint in 2-3 years. There are some paints for galv iron like resene galvo one but the area around the rust will be sticking to the already painted surface not the iron.

Also how do I prevent it rusting through I read that I should use oil based paint but also read I should use acrylic which is water based.

The paint already on the roof seems to be stuck well (not flaking).

timmmay

20578 posts

Uber Geek

Trusted

Lifetime subscriber

#3176676 1-Jan-2024 12:27

When that happened on my old roof I replaced a few iron sheets. A few years later I replaced the whole roof.

tweake

2391 posts

Uber Geek

#3176683 1-Jan-2024 12:49

galv paint.

i suggest having a look on the underside of the iron, odds are its really rusty.

mdf

3513 posts

Uber Geek

Trusted

#3176703 1-Jan-2024 14:12

If is just surface rust, you can wire brush it, then use Rust Converter (CRC brand has the Geekzone Seal of Approval™️): https://www.geekzone.co.nz/forums.asp?forumId=141&topicId=311159). Personally I would then overcoat with a Zinc rich primer (see that previous link, e.g., CRC Zinc It). Then two top coats of acrylic roof paint, e.g. Resene Hi Glo.

I've used the Resene roof priming stuff (e.g., Galvo One) and its okay but it's not my favourite stuff to use (and I am normally a bit of a Resene Fanboi). Galvo one is essentially oil based, but you should still topcoat with acrylic. Acrylic paints are much more flexible, which is what you need for high sun areas like roofs.

If it is rusted all the way through, the best results are clearly going to be from replacing the rusted sheets. You can patch small holes with something like Denso Tape before overcoating, but read the instructions.

Some pointers for roof painting:

Consider a safety harness.
Clear up and treat all rust as above/following directions on the can, especially regarding overcoating.
Thoroughly clean all lichen, moss, gunge etc. with bleach/30 seconds etc.
Check nail/screw heads to see if any are lifted. If old lead head nails, I would recommend prying up (prybar plus a half round will work well on corrugated roofs) and replacing with screws with nylon washer. Spray some more Zinc it in the hole before you put the new screw in.
Check any other penetrations, e.g., pipes or vents.
Disconnect your downpipes. Wash your roof with a house cleaner (e.g. https://shop.resene.co.nz/resene-roof-metal-wash). Alternatively the raven telescoping brush and soap sticks worked well for me. Washing is especially important if you live anywhere coastal! I live on Wellington's south coast, and it took me about a week to paint my roof. I washed it down every day. It is a pain but needed.
Use a brush to spot prime all the screw/nails heads and in and around flashings. You don't need to be neat, but do need to be thorough.
Only paint first thing in the morning. If your roof is too hot to put your hand on, it is too hot to paint. The paint will flash dry before it can bond and will flake off. It's actually better to do it in the shoulder season than the height of summer. You can get hot weather additives to slow the drying time but they aren't miracle workers.

My roof painting (work in progress shot).

I picked a metallic "aluminium" colour. I thought it would reflect more light so make the roof space cooler - that bit worked but there is a metallic flake in the paint that did not like the roller at all so I ended up brushing the whole roof. Two coats...

Bung

6478 posts

Uber Geek

Subscriber

#3176887 1-Jan-2024 20:55

OT question, why is the left-hand pipe/vent capped off?

mdf

3513 posts

Uber Geek

Trusted

#3176888 1-Jan-2024 21:01

Bung:

OT question, why is the left-hand pipe/vent capped off?

It was before my time, but best guess there was some kind of rooftop header tank at some stage and It was was easier just to put a cap on the feed and leave it for the next poor sap (yours truly) to deal with it properly. Those penetrations leaked and needed fixing properly so I got to pay for the plumber to remove them.

angski

59 posts

Master Geek

#3176949 1-Jan-2024 22:15

Do it once and do it right. You have two options, Either Dulux Metal shield epoxy enamel or Resene Alumastic. I had used both for rusted area (not for roof). The Dulux version is easier as it is like an oil based paint. But the Resene one is harder to use. It is a two part paint and you got to mix it together before you can use it. Then you only have 4 hours to finish the job or else the paint will harden. I prefer the Resene paint as it is rock solid after four hours. If you do want to use the Resene paint, make sure you ask the Resene shop to shake the paint for you.

Jase2985

13459 posts

Uber Geek

ID Verified

Lifetime subscriber

#3177131 2-Jan-2024 15:49

mdf:

Bung:

OT question, why is the left-hand pipe/vent capped off?

It was before my time, but best guess there was some kind of rooftop header tank at some stage and It was was easier just to put a cap on the feed and leave it for the next poor sap (yours truly) to deal with it properly. Those penetrations leaked and needed fixing properly so I got to pay for the plumber to remove them.

possibly the header pipe for a low pressure hot water cylinder that's been changed to a mains pressure one ?