Bob, see the thread underneath this one,'emulsion as wood undercoat.'

Acrylic primer is a primer/undercoat, so the theory is you put one coat on as a primer and another as an undercoat, then gloss it.

Me, I would apply a coat of shellac knotting to the knots followed by a coat of oil based primer. On top of this I would use an oil based undercoat and then an oil based gloss coat.

The very least I would do if I had to use acrylic is one coat of acrylic primer, one coat of oil based u/c and one coat of oil based gloss.
Acrylic being water based raises the grain of the wood and does not flow out very well, so leaves more brush marks than an oil based paint.
Leave the acrylic to the builders who need to slap some paint on and go home to watch Richard and Judy.

Gotya. Thanks very much. I'll be off to the shops then.

While on the primer subject I've got to fireproof a metal beam, I've got some intumescent paint which say's use a good quality primer. Any recommendations for a 'good quality' primer that the average Joe can buy. From Gibbs & Dandy, Travis Perkins or even Screwfix preferably.