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Best rust converter for tiny rust spots on painted steel roofing?

etting | Posted in General Questions on

One of my new, bright white Classic Rib roofing panels has roughly a dozen widely scattered, tiny spots of rust, possibly the result of little grains of steel produced when I predrilled all of the panels, although I brushed them away and don’t know why only the one panel would be affected.  Each spot produced a small stain that washed away; the actual, dark rust spots themselves are less than 1/16″ diameter.  I looked at them with a high-powered magnifier, and they have penetrated the paint, but not evidently any farther.  Rather than replace the whole panel, which would be expensive, I thought of applying some rust converter and then painting; bright white is easy to match.  There’s quite an array of rust converters out there.  Which would you recommend for this application?

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  1. Trevor_Lambert | | #1

    The first thing I'd try is Evapo-rust. It's a remover rather than a converter. Soak a paper towel and place it over the area, and cover than with plastic wrap to keep it wet. If the rust is not thick, it shouldn't take long, but I'd leave it for a day anyway.

  2. etting | | #2

    Thank you, Trevor. Given how thin the 29 ga steel is, I thought converting the tiny spots of rust to a stable form would be better than removing them, as it would leave more thickness in place. If a remover left any rust at all, I worry that it might remain active. What advantage do you see in removing versus converting?

  3. Trevor_Lambert | | #3

    If the rust has not spread under the painted surface, I'd say the chances that it's removed a lot of the base steel is very low. And if it had, "converted" rust has minimal structural integrity.

    I use rust converters as a last resort, mainly if I can't get access to the part, or I'm only going for a modest extension of useful life. In order of preference:
    1. Mechanical removal (grinding and sanding until zero rust remains)
    2. Chemical removal (ideally no rust remains in this case as well)
    3. Rust conversion

    The weakness of rust conversion is that you don't know if you got all the rust. The surface may look fine, but there might be rust underneath. I think the ability for that stuff to penetrate deeply is very limited. So it's a kind of paradox; the thinner the rust, the better a rust converter works, but in that case removal is also easy and preferable.

  4. etting | | #4

    Thank you again, Trevor. It makes sense that with rust remover, you can see whether it has removed all of the rust, whereas with a converter, you can see only the surface. Nothing on the EvapoRust website, however, addresses using it on a painted surface, and several of its reviews at Amazon indicate that it's great for soaking tools, etc., but less so for something that needs to be painted. Some say the bare metal will start to re-rust too quickly, others that it leaves a residue that's hard to remove, and others that it removes paint. Some of the converters claim to leave the surface primed and ready for paint, and I would think that if they worked on anything, they'd work on such tiny specks of rust as these. I hope others will weigh in, as I haven't been able to find any reviews or discussions that help me decide which specific product would be best for this application.

  5. Trevor_Lambert | | #5

    Pretty much every rust treatment leaves some kind of residue to be removed prior to painting.

    Why not go at the spots with a small piece of closed sand paper, see if you can get to bare metal? If so, apply a small amount of primer, then the touch up paint.

    If that seems like too much work, use the rust converter as you originally suggested, but you'll have to clean the surrounding paint prior to touching it up.

    Sorry, I have no recommendations on specific paints. Make sure it's intended for metal.

    1. etting | | #6

      Thank you, Trevor. The manufacturer's only advice was not to try to sand away the rust, as the metal is too thin. Also, sanding would expand the area of damaged paint by a factor of at least 10. I'm hoping someone will be able to recommend the best rust converter for this application.

  6. etting | | #7

    For the benefit of anyone else who finds tiny rust spots on their painted steel: When I scrubbed the panel with a stiff nylon brush as widely recommended, it removed the rust stains around the tiny specks of rust, but not the specks themselves. Using a wooden toothpick, however, I was able to remove the actual rusty bits of steel, which turned out to be embedded in the outer layer of paint, but no deeper; with the specks gone, the paint remained intact underneath, and the panel looks as if they had never been there. It was meticulous work, but well worth it.

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