Does it need painting/re-painting?
A freshly painted white milestone with crisp black lettering looks very smart. Before deciding to paint your milestone, take care to check that it was painted in the past. Cast iron will almost certainly have been painted, but stone may not have been.
If the stone was painted, it was probably done with limewash, which is sacrificial, ie it weathered away but protected the surface of the stone, although this had to be repeated frequently. However, limewash doesn't work well on gritstone or granite; it doesn't bond well with sandstone either and repeated coats will begin to erode the inscription.
Are there traces of old paint on the milestone? Are other milestones in the same group painted? Agree with your Highway Authority contact that the milestone should be painted; ideally also agree a future maintenance plan because repainting cast iron only lasts 7 – 10 years at best and shabby, rusty paintwork does not look smart at all.
Can the milestone be taken to a workshop or will it need refurbishing in place?
It is easier to deal with a cast iron milestone indoors, although they are usually set deeply into the ground. The layers of paint and primer required are easier to apply in dry conditions. If it’s completely rusted cast iron, sandblasting might be worth considering, although milestones frequently retain a glass-like residue from the original sand-casting process and this should not be removed as it acts like a base primer. However, after sand-blasting it is essential to apply the first priming coat immediately, otherwise the surface will oxidise.
By the wayside
Is the location likely to be dangerous for you or anyone else, pedestrians, passing vehicles? Do that thorough risk assessment! If the milestone is within two metres of the road edge, you will need to set up road signs and cones to warn road users of your presence on the verge. Wear a hi vis vest. Once again, your friendly contact at the Highway Authority can advise on your particular milestone, but expect to make at least four or five visits to carry out the work required to strip and paint a cast iron milestone.
Black enamel paint can be used for lettering both stone and metal. When having new plates or fingerpost arms cast, we have tried using powder coating by companies specialising in this, but the results have generally been unsatisfactory.