Small diorama and one question

So this is a small diorama of a banshee attacking a couple of marines in cover, hope you enjoy but before you go i have one question. Ive been wanting so make some really nice custom figures but not only with paint, i want to add a wash, so what material should i use for that? And is there any specific things i should know when adding a wash? Also to make sure the paint doesnt scrape of a custom figure, would sealer be good to use on it? Thanks for reading this lol cya guys soon