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I'm thinking of repainting the metal parapet on the balcony, paint has already mostly come off, or it is peeling off (and the metal is also starting to rust). Every description starts with scraping off rust and old paint which is fair enough.

Several years ago, I had repainted a smaller, similar parapet, and to get all the paint off was a long and painful process, because most of it was still stuck to the metal.

What I'm not sure about: are you supposed to get all the old paint off, or only the part which is peeling away, and hence is easy to remove?

(In other words: can you partly remove the old paint, and then paint over the remaining patches, which are still stuck to the metal? If yes, what are the disadvantages of doing so?)

Similar question, but about wooden frames. (Maybe answer is different for metal?)

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The most important part of painting is preparation.

You will want to remove all loose and flaking paint and you will want to remove all rust, which will probably involve scraping and grinding with power tools.

You will want to carefully inspect any paint that appears to be well adhered. You want to make sure that it is still stuck and that there isn't rust underneath it waiting to bubble up and pop the paint off. If you do find well adhered paint on good, solid metal, feel free to leave it. You'll probably want to give it a light sanding, just to make sure that it's got a "tooth" for the new paint to bite into, but if the rest of the paint is that old, it's probably pretty well worn, too.

Additionally, for any paint that is well adhered, you'll want to sand the edges to create a smooth transition from bare metal to existing paint. If you leave a sharp edge at the existing paint, you'll still see it when you apply the new paint.

When you start painting, be sure to use a good primer to ensure a good grip on the bare metal so your paint will stick longer. I don't think it would be an issue, but make sure the primer you pick will adhere well to existing paint as well as bare metal - a question to ask at the paint store.

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    And by the way, on metal, you should use an icky, stinky alkyd paint - the canonical metal primer readily available is Rustoleum Rusty Metal Primer. Do not use a latex or waterborne alternative - they won’t perform nearly as well. You use natural bristle brushes with this, and get cheap chip brushes - then you can just throw them away afterwards. It also helps if humidity is extremely low. – Harper - Reinstate Monica Jul 4 at 6:57
  • Sadly, @Harper-ReinstateMonica, for those of us in the rust belt the only time the humidity even approaches extremely low is in the dead of winter when it's far too cold to apply paint of any sort. :( (Says the kid who grew up in ID where summertime humidity of 20% was considered miserably humid.) – FreeMan Jul 4 at 13:59
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    Well if it’s outdoors there, just wait until the sun is blasting on it and making it 150 F. It’ll be dry! – Harper - Reinstate Monica Jul 4 at 14:36

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