The 40,000 annual calls to BAL's free technical advice line raise questions about every aspect of tiling. Recent callers have been asking the experts about priming, with such queries as, "Do I always need to prime?"
Priming matters: it can promote increased bond strength, can seal the surface to make it stronger and more stable and can offer protection against the penetration of atmospheric moisture. Sometimes priming is not strictly needed, but this is uncommon and it is rare for priming to be completely unnecessary. The most appropriate priming agent — either epoxy-based, acrylic or synthetic rubber latex — will depend upon the circumstances.
For dusty or highly-polished plaster walls, thoroughly brush down with a nylon scrubbing brush or similar to remove surface dust and open up the surface. Then apply a priming agent in accordance with the manufacturer's instructions. BAL Bond SBR, for example, is diluted with four parts water to one of primer for use with dispersion adhesives, or in a 2:1 ratio for cement-based adhesives. BAL Prime APD, however, is diluted 1:1 with dispersion adhesives, but used undiluted with cementitious adhesives.
Priming may be unnecessary if a plaster surface is not over-trowelled or polished, if surface dust can be removed and if a dispersion adhesive is being used. Using a dispersion adhesive on a primed wall may slightly extend the drying time.
With timber walls, follow the procedures outlined in both BS 5385 Part 1 and the TTA document on tiling to timber substrates. All exposed edges and the backs of boards, but not faces, should be sealed to protect from distortion by atmospheric humidity changes. Normally, undiluted BAL Bond SBR is recommended.
The same recommendations apply to timber floors. Priming the face of tongue-and-groove softwood floorboards is essential prior to applying BAL Fastflex adhesive, to prevent moisture distortion within the boards caused by the exchange of moisture with the adhesive. If required, priming can be considered on overlaid timber floors.
Anhydrite (calcium sulphate) screeds must dry sufficiently before priming: i.e. less than 0.5% water by weight, measured by a Speedy moisture detector. Priming is essential (two coats of undiluted BAL Prime APD are ideal) before tiling to avoid the risk of ettringite formation. This salt can be destructive to cement-based adhesive systems.
Other applications with specific priming needs include tanking, where pre-sealing with a product like BAL Prime APD is recommended. Slurry bonding coats can be used for screeds and renders, as a waterproofing coat for concrete tanks, comprising a simple 2:1 mix of BAL Bond with ordinary Portland cement, applied in three coats.
The need to prime can depend upon the choice of adhesive as well as the substrate. When using BAL Gold Star or BAL Rapidset on existing vinyl tiles, priming can aid to bonding. If BAL Single Part Flexible or BAL Rapidset Flexible were to be used on the same surface, priming would not be essential.
In the long run, it is unlikely that unnecessary primer will cause damage, but applying it will cost you both time and money. When in doubt, as with any technical question regarding tiling, a reputable manufacturer should be able to offer impartial advice.
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To prime or not to prime, that is the question
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Originally Posted by BTB 
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