Painting over rusty plaster bead

[NeilPalfreyman]

I'm decorating a house that has a few areas where the metal plaster bead on the wall corners has surface rust. In almost all cases its's superficial and easily sands off leaving bright metal underneath.

I have tested the walls with a damp meter and there is no sign of dampness in the walls but I'm not sure how to treat the corners to stop the rust breaking through the paint we're intending to put on (matt emulsion)

In addition in one place there was a (now fixed) leak and the rust is much deeper. Not deep enough to cause the bead to disintegrate but deep enough to not be fully removed by sanding.

Any suggestions of what to do in each case (if different?)

 

[FrogParty]

Since no one else has commented here's a go...

In a similar situation decorating with rusty corners beads, builder told me they usually paint over with oil-based undercoat. Failing that use a (more expensive) stain stopper paint or one of the Zinnser range, some are much quicker drying than u/c if time is an issue. Anyway I had a can of oil based u/c in the house so applied that to the areas and painted the emulsion over it once dry. Seems to have worked.