When it is desired to a have a tiled finish to a plasterboard surface, depending on the usage of the room, there are certain steps to take.

For tiling to plasterboards (not high humidity or wet areas) on a partition or lining, the stud centres must be reduced to maximum 400mm centres, the joints reinforced with paper tape and filled (not the full tape and joint process) prior to a suitable tile adhesive (by others) being applied for the tile finish.

For spaces of low to high humidty (washrooms, toilets, kitchens, bathrooms), Siniat Moisture Board, Fire MR board, LaDura or Aquapanel are recommended again with the supporting substrate at max 400mm centres. Joints to be reinforced with paper tape and filled with the correct jointing compound and then a suitable waterproof tile adhesive (by others) to seucre the tile finish.

For spaces of severe humidity (swimming pools, spa, wet rooms, public showers), only Siniat Aquaboard is recommended to be used. Again the framing substrate must not exceed 400mm centres. Board joints reinforced with paper tape and filled with Siniat Aquamix. The surface then should be pretreated with 2 undiluted coats of Siniat Drywall Sealer. Siniat Aquastrip applied and the junction of the base and floor bedded in Siniat Drywall Sealer. A suitable waterproof tile adhesive (by others) then applied to receive the tile finish. 

So in summary, only the severe humidity area requires a pretreatment of 2 coats of Siniat Drywall Sealer.

Also note, plasterboards can take a maximum of 32kg/m2 of tile weight, although Siniat Aquaboard can take 50kg/m2.

You can also refer to the following documentation on the Siniat Website:

Drywall Manual Annex / Siniat Aquaboard Brochure / How to: board bathrooms and wetrooms