what do do with a rusty cast iron step
June 22, 2019 6:05 PM Subscribe
I purchased a 120 year old iron step. It is very substantial (likely 100+ lbs) but has a thick, pitted coating of rust. I tried scrubbing one area, with little impact. Sandblasting and powder-coating would cost at least $300. I plan to use this as an outdoor step. Would it be feasible for me to just coat it with rustoleum paint and use it, or do I really need to get it sandblasted?
My goal is for the step to look decent and to remain solid, I don't need (or want) it to look perfect and new.
My goal is for the step to look decent and to remain solid, I don't need (or want) it to look perfect and new.
Best answer: Avoid sandblasting if you can. There are products and coatings designed for things that have a little rust.
Bear in mind, cast iron is usually pretty rust resistant, but if salt was ever involved, that's a different story. Salt gets into the grain of the metal. It can be removed by electrolysis, but I suspect that's more than you want to get involved with. If it has salt in the metal, it will continue to rust. You can slow it up though.
Get all the loose rust off with a wire brush. Then use the right products.
People swear by this stuff, I've never used it.
(I used to do cannons etc that came off the sea floor. We had to "cook" them in the electrolysis vats for l o n g periods of time to get the salt out. )
There are other products similar, "Rust reformer" is often sold at auto parts stores. What I used is Ospho. These products contain dilute phosphoric acid. No, it won't hurt you!
Ospho is usually available at a good paint store. It's a fully liquid (watery) compound, you brush it on, the phosporic acid reacts with the ferric oxide to form iron phosphate, which is very stable. Ospho is a bit tricky, you just want to wet the surface, no standing liquid. And don't let water touch it till you've got it painted.
The "Rust reformer" has a poly something or the other plastic with the phosphoric acid, so it treats and primes in one application, and you can paint right over it.
With the ospho, we then used at least two coats of a zinc rich metal primer, then a couple of top coats.
You must get all the loose rust off. If a hand wire brush won't get it, a wire brush on a drill motor or angle grinder is called for.
posted by rudd135 at 6:18 PM on June 22, 2019 [3 favorites]
Bear in mind, cast iron is usually pretty rust resistant, but if salt was ever involved, that's a different story. Salt gets into the grain of the metal. It can be removed by electrolysis, but I suspect that's more than you want to get involved with. If it has salt in the metal, it will continue to rust. You can slow it up though.
Get all the loose rust off with a wire brush. Then use the right products.
People swear by this stuff, I've never used it.
(I used to do cannons etc that came off the sea floor. We had to "cook" them in the electrolysis vats for l o n g periods of time to get the salt out. )
There are other products similar, "Rust reformer" is often sold at auto parts stores. What I used is Ospho. These products contain dilute phosphoric acid. No, it won't hurt you!
Ospho is usually available at a good paint store. It's a fully liquid (watery) compound, you brush it on, the phosporic acid reacts with the ferric oxide to form iron phosphate, which is very stable. Ospho is a bit tricky, you just want to wet the surface, no standing liquid. And don't let water touch it till you've got it painted.
The "Rust reformer" has a poly something or the other plastic with the phosphoric acid, so it treats and primes in one application, and you can paint right over it.
With the ospho, we then used at least two coats of a zinc rich metal primer, then a couple of top coats.
You must get all the loose rust off. If a hand wire brush won't get it, a wire brush on a drill motor or angle grinder is called for.
posted by rudd135 at 6:18 PM on June 22, 2019 [3 favorites]
I have successfully used electrolysis on several iron pieces, provided you have or can get a big enough container to put it in. Evaporust also works well if you are able to soak it.
posted by Hal Mumkin at 6:30 PM on June 22, 2019 [1 favorite]
posted by Hal Mumkin at 6:30 PM on June 22, 2019 [1 favorite]
I have had good luck using the aerosol Rustoleum "Cold Galvanizing Compound" paint, it has a ton of zinc in it. So, basically a +1 on the "zinc rich metal primer" recommended by rudd135. Also +1 on putting in the grunt work to get all the loose rust off for better results.
posted by Larry David Syndrome at 7:49 PM on June 22, 2019
posted by Larry David Syndrome at 7:49 PM on June 22, 2019
You could also attempt to restore it in the same way that people restore very rusted cast iron pans. After that, you can coat it.
posted by quince at 8:27 PM on June 22, 2019 [1 favorite]
posted by quince at 8:27 PM on June 22, 2019 [1 favorite]
Best answer: Hi, I do things with rusty old boats.
Seconding points above about wire brushing and phosphoric acid based products as a rust converter.
Just a warning though that where a lot of people go wrong is not getting enough of the rust off. The phosphoric acid will only penetrate and convert an extremely thin layer of it, think microns. Any thicker than that, you can treat the rust and paint and get a good looking result, but then the rust will start bursting out from behind the coating soon after.
If you're dealing with a flat object, a wire brush on an angle grinder is good enough; doing it by hand may not be.
If the object is an ornate shape with fiddly grooves, it may be really tricky to do the job properly without blasting.
If you're not in a rush, a good approach is to do the job as best you can and then apply one of the phosphoric products that includes a latex/polymer carrier to form a coating and act as a primer. Then leave it outside for a couple of weeks, see if and where it fails, and rework those areas. Repeat until it stays sealed, then apply paint.
posted by automatronic at 3:10 AM on June 23, 2019 [1 favorite]
Seconding points above about wire brushing and phosphoric acid based products as a rust converter.
Just a warning though that where a lot of people go wrong is not getting enough of the rust off. The phosphoric acid will only penetrate and convert an extremely thin layer of it, think microns. Any thicker than that, you can treat the rust and paint and get a good looking result, but then the rust will start bursting out from behind the coating soon after.
If you're dealing with a flat object, a wire brush on an angle grinder is good enough; doing it by hand may not be.
If the object is an ornate shape with fiddly grooves, it may be really tricky to do the job properly without blasting.
If you're not in a rush, a good approach is to do the job as best you can and then apply one of the phosphoric products that includes a latex/polymer carrier to form a coating and act as a primer. Then leave it outside for a couple of weeks, see if and where it fails, and rework those areas. Repeat until it stays sealed, then apply paint.
posted by automatronic at 3:10 AM on June 23, 2019 [1 favorite]
Response by poster: Thank you all! I'm off to the hardware store...
posted by arnicae at 8:00 AM on June 23, 2019
posted by arnicae at 8:00 AM on June 23, 2019
If you can find a container that would let you fully submerge your step without needing too much expensive liquid to do that in, you might have some luck with a urea hydrochloride based formulation like Eastwood Rust Dissolver.
posted by flabdablet at 10:44 AM on June 23, 2019
posted by flabdablet at 10:44 AM on June 23, 2019
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