JohnH wrote:
I painted some rusty old pipes. I coated them in a milky white substane to
neutralise the rust, then an undercoat and topcoat. The rust dabbled
through, unfortunately.

You'll need to get some Kilz from wherever you buy your paint. It goes
on like white paint, but provides a shield against any kind of existing
staining such as rust or water stains which can bleed through ordinary
paint. Then you paint over the Kilz.