I use bal APD or BAL bond sbr....there plenty other makes out there....
I always advise priming..especially if using cement based adhesives or you will get a reaction between the adhesive and the Gypsum in the plaster....
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.
BAL Bond SBR
A waterbased polymer with a wide variety of uses in the construction industry, particularly as an admixture and bonding agent for cement and concrete applications. Once cured, the polymer is water and frost resistant. BAL BOND SBR can be used as a slurry coat when mixed with hydraulic cement or gypsum plaster, as a multi-purpose admixture for cement:sand mortars, concrete and gypsum plaster; as a brush applied primer, sealer or dustproofer.
Suitable for use on cement:sand screeds, toppings or renderings, plastering, waterproofing, bonding, concrete repair, priming and sealing.