I have painted out a few garages internally straight onto the bricks/blocks. They seem to have held up for years with no problems. The bricks are porous so totally suck the paint in, it isnt ever going to come off!
My method is to use very cheap normal emulsion. The bricks will soak it up rapidly so the cheaper the better. I have used either leyland 15L tubs from screwfix £15 for 15L, B&Q Value 15L (even cheaper than £1/litre i think), and also Crown 15L tubs when they have been on offer at 2 for £30 (£1/litre). The Crown and leyland paint is way too good for the job really, but at £1/litre you cant really go wrong. A single 15L tub will likely do you for a whole single garage.
I dont bother watering it down, as i say at £1/litre who cares and the thicker the better as far as ive always been concerned.