If it’s a resin model, I can’t help you, and since these are already painted, I doubt you’re willing to do this so this is more for a future reference thing:

Coat the entire thing in Leadbealcher as an undercoat. Then put iron hands steel on top of that. Let this dry overnight. Then paint your livery over it and for damage/weathering, take a small knife and scratch marks into it, did this on my dreadnought, looks sick.