In article , Huge writes
The steel (?) edging buried in the plaster round the window (does this have
>a proper name?) has gone rusty. I want to emulsion over it - how should I
>stop the rust from bleeding through the emulsion?
>
Is it superficial rust, on the odd spot where the galvanizing has been worn or sanded through? If it is more wholesale then you may have a problem with damp in that spot which will cause the whole strip to rot and blow off the plaster.
If superficial, I would treat the rust with jenolite or another phosphoric acid based rust treatment, following the instructions on the bottle, then prime with oil based primer and oil based top coat back about 2" either side of the capping strip. The 2" band of gloss should hold back any bleed through from the deeper stuff for a bit. Overpaint with emulsion after a couple of days.