masonry paint and emulsion does not have undercoat. However, bare bricks, and especially blocks or concrete, need to be treated with one, or preferably two, coats of the paint thinned with water. It will soak into the surface for proper adhesion, and will kill the suction.
You will know when you have done it because the wall will no longer suck the paint off your brush.
Masonry paint is more hard wearing and easier to clean. If necessary you can hose or scrub it.
Once you paint brickwork, you will have to paint it for ever. Bare bricks don't need painting.