Cast iron plates or a cast iron mile marker needs a different sort of paint to a stone milestone.
Cast iron, unlike steel, will not usually rust away in a quiet roadside location. It will build up a protective (oxide) layer on which you can paint. However, by busy roads with lots of grit and salt spray or other chemicals, cast iron will rust and this must be removed completely.
Think carefully before you decide to remove the old paint. It may be providing a protective covering. It is part of the history of the mile marker. It will make a mess on the verge which will be difficult to clean up. Take piles of newspaper, which are easy to remove and bundle into a bin liner afterwards. Alternatively, cardboard or an old sheet are less likely to blow away. It will be difficult to remove all the old paint. Usually, it is best to take off any loose or flaking paint, clean the surface with water and leave it to dry, then paint over the old paint. You MUST get the rust completely out of the lettering corners otherwise it will leach through in a matter of months. And only do the work when the weather is warmer than 5 degrees because high humidity will cause a bloom on solvent products.
Chemical paint removers should ideally not be used at the roadside for environmental reasons, but if the milepost is very rusty, Nitromors works. You will need copious amounts of water to wash it off. Use plenty of newspaper to contain the residue and let the surface dry completely before attempting the priming coat. Shot blasting will damage the metal surface – sand blasting doesn’t work by the roadside unless you can get the priming coat on within a few minutes; although hand-held pneumatic equipment is reasonably cheap, it needs to be powered which means having a compressor. If using hand tools to remove old paint, the metal of the tools should be softer than the cast iron.
Exposed bare metal should have at least one coat of zinc-based or red oxide primer, followed by two coats of a metal paint, such as a white tractor enamel, eg Promain Teamac Tractor Enamel Rapidry*. These paints are usually cellulose based and require chemical solvents for brush cleaning. You can wrap used brushes in a plastic bag and clean them when you get home. DIY paints such as Hammerite may be used in exceptionally sheltered conditions but are not suitable in exposed environments; they require immaculate preparation too - you can't just spray them onto a rusty cast iron surface. Mixed results have been obtained from epoxy paints; unless the metal is completely stripped prior to priming, the rust will rapidly leach through. DO NOT USE HOUSEHOLD GLOSS – the metal can’t breathe and the paint surface will deteriorate within months.
Some cast iron work will have flat faced lettering, which is easy to paint black with a sponge or small roller, although this will not work well if the lettering has a triangular or round profile. Use a black enamel paint.
When having new plates cast, we've tried using powder coating by companies specialising in this for outdoor applications, but the results have generally been unsatisfactory and rust has started to show within months.
*Promain’s recommended system would be: