Originally Posted by robbo

Bal only recommends priming the rear and sides when tiling onto plywood NOT the surface to be tiled.

Any priming is usually done with a primer like this
BAL Prime APD
A modified acrylic polymer dispersion for use as a primer for a variety of backgrounds/bases prior to fixing ceramic tiles, mosaics and natural stone and prior to the application of levelling compound and the BAL WP1 TANKING SYSTEM. BAL PRIME APD isolates products containing calcium sulphate such as gypsum plaster and anhydrite screeds from cement-based adhesives, thus minimising the risk of reaction between the two materials.

Also suitable for use on cement:sand rendering, blockwork, plasterboard, cement:sand screeds and timber. It also provides surface strength to friable surfaces and reduces the porosity of porous backgrounds/bases, thus preventing the formation of air bubbles and prolonging the flow time with levelling compounds and extending the open time of tile adhesives. The primer is blue in colour to help ensure that a complete and even coating is obtained.