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wrote:
Jim Scott wrote:

Are proprietary 'Stain block' paints any better than oil-based undercoats?

Yes. Oil based undercoat will block some stains, stain block will
handle more.

NT

The only stains I've come across are from old water leaks. I guess the
reason they're always brown is either that they carry rust from fixings
in the masonry or oxides from the bricks. Maybe John Schmitt can
enlighten us on that.
Any oil based paint will block these, but any water based coating won't
IME.