Rust Marks Coming Through Plaster

I'm decorating and there are rust marks in one corner coming from the galvanised plaster edging bead.

There was water ingress in that corner, but I've re-roofed it last year and I'm now decorating. Would a coat of old gloss paint seal it? Or do any of you guys have any magic tips to prevent the rust showing through? I may resort to dry lining the whole room at some later date, as I want to put in a shower W.C, basin and add a couple of windows in the longterm.
The room is part of an extension 15' square with a flat roof.
Del
P.S I've got the mice out of there now, they were coming under the pation door which was installed by contractors a few years back, they never caulked under the sill.