If deciding to paint on metal with acrylics the most common choice would be aluminum in order to avoid the problems associated with more corrosive options, like steel or copper. Along those lines, the best introduction to the topic would be an article Mark Golden wrote for our Just Paint newsletter:
as well as this one on preparing Dibond, an aluminum composite panel:
Especially if going outdoors, and/or choosing to work on raw aluminum, you are best off preparing the metal with high end commercial primers made for that material. As the article lays out, for aluminum the best options will involve three steps of degreasing, etching, and then application of a conversion coating. Because this involves a lot of work, we strongly recommend going with a pre-prepared aluminum panel such as Dibond, which comes with multiple surface coatings - including clear coats that preserve the look of brushed raw aluminum - which will allow you to paint on top of them will far less prep. In fact you could get away with simply scuffing the surface, cleaning with isopropyl alcohol, then using an acrylic gesso if painting indoors or using something like Sherwin Williams' DTM Bonding Primer or XIM's UMA. Both mentioned in the articles above. After that, then you can proceed with acrylic paints as you would normally. If going outdoors, you would need to be more selective about which colors you use. Here is a list we give for outdoor murals which are the most durable and lightfast in exterior conditions:
While the list is based on testing of our own paints, in general the recommendations should hold for most other brands as well.
Once done, then you should protect the piece with a removable varnish or topcoat. If going outdoors, the gold standard would be a two-part automotive urethane, which an auto body shop can apply. If that is not an option, then a solution acrylic varnish - such as our MSA Varnish or our Hard MSA Topcoat - would be alternatives. Liquitex's Soluvar would be a similar choice. However, none of these are as physically durable or chemically resistant as the auto clear coat. If the piece is indoors, then your options are broader and even a water based acrylic varnish would be a viable option.
We hpe this is helpful. In truth a lot depends on the particulars of the piece - whether indoors or out, - and your expectations about longevity. If the piece only needs to last a decade or two, that is different than wanting something that is permanent. Even the best metal sculptures outdoors - including a lot of Calders - end up being regularly repainted because the environment is just so challenging. But indoors your chances for the initial coatings lasting a long time are much better.
If you are planning to use a different type of metal, or having further questions, let us know.