There is an (100 yer old) internal cast iron soil pipe from upstairs
runs vertically through our utility room. It has been painted over the
years with a few coats of emulsion. We're redecorating so I stripped
off the old paint and cleaned it up.

When cleaning it up, I noticed a few small (10mm max) blisters on the
old paintwork with pockets of damp/rust behind them. Regardless, I gave
it a couple of coats of metal primer with intending to paint it with
the same emulsion as the walls when I get that done (this weekend).

However, after a week, there are tiny bead of rust/water has come
through the undercoat. There's not enough to form a drip, just a small
brown patch coming through the paint. It's not a big problem, and I
certainly don't think it worth replacing the pipe - more of a cosmetic
issue really.

Can anyone suggest a teatment that would keep it blemish free. What I
don't want to do is put the emulsion on and have the brown spots come
through that too.

Thanks.