Falanx

Essentially, it's a fundamental of polymer chemistry. Even the most rigid, cross-linked, three-dimensional polymer structure effectively terminates in lots of places rather quickly, so what's left holding these ten-thousand molecular weight monsters together is dipole interactions and Van der Waals forces. Essentially, static. Gas molecules, and tiny liquid molecules can elbow their way through with enough head of pressure. That's why beverage bottles have a shelf life.

More... the porosity is capitalised on. The pores allow ion transport and therefore maintain the continuity. The paints are designed to go down in a film thickness range x-y and in that range there can be assumed a surface-connecting porosity range usefull but not excessive.

That would be.. (etch primer if it needs it), strontium/potassium-zinc epoxy primer, topcoat. Yep.

You want maximum toughness, I'd say polyurethane topcoat and a good sacrificial epoxy primer. I had to get a Mil Spec PU paint off once. It took a oxy-propane torch and glass bead blasting. No joke. Two HOURS. But it's once again, all down to the prep. You make sure the subsrate is clean and keyed, you take your time with the primer and then you key it again, unless you're handy with putting down a final powder layer on the primer. Be aware that the two are partially mutually exclusive. A wet-coat car paint is real easy to use, but not particularly hardwearing by comparison. A PU paint is a bit of a pig to clean up and strip out of spray tools, but it won't come off, and the chemically resistant nature as a paint is reflected in its behaviour as a film coating.