The bricks will change colour to a shade or two darker PVA does seal but it dries to soft finish For something way tougher, try Bona Chem Traffic acrylic floor varnish in dead matt Or Leyland SDM do an Acrylic Dead Flat decorators varnish (in a turquoise container) The decorating shop up near the cemetery end of West Norwood high st sell it too, + free car park at the back. Or Whites on Brockley rise are good, see John the Irish guy in there, most helpful. Don't roller it as it creates micro-bubbles that pop as they dry, leaving micro-craters. Thin the first coat with water and brush on with an acrylic chinex bristle or similar synthetic bristle brush. 2nd coat will go on way quicker then the first. Use green liquid Flash floor cleaner to assist when cleaning the brush, it contains ammonia which is the solvent for latex/acrylic Stir not shake for acrylic products (same bubble issue)