I've not used that humbrol stuff for years, it was horrible and lumpy last time I did. I'd use the RCS stuff, top quality product ! Also you need a few coats and I doubt they'll be enough in the humbrol bottle. Paint the whole shell with it, then cut out the area's you want to paint, cut all of them out, then in order of darkest colour first go from dark to light spraying in between, you should be fine then
