If you don't prepare your wall then no matter how good you are at plastering it could all go wrong!
Controlling the suction is crucial to obtaining good results. If the wall you are plastering is porous then the plaster will dry and firm up too quickly (due to the moisture being sucked out of it) before you have had a chance to work it. It doesn't matter how good your technique is – if the suction is not controlled properly then it can be impossible to achieve good results.
To find out if the wall you are about to plaster is too dry
When soaking with water care should be taken to apply it evenly to ensure even drying across the wall.
Prepare your wall - Adhesion
If you are skimming a surface that has been painted or is not very porous (low suction) then adhesion may be a problem. PVA should be applied.
A background that provides no suction at all must be properly prepared or the plaster will not stick. The use of a bonding agent such as PVA will need to be painted to the wall to ensure adhesion. PVA should be diluted with water (follow instructions on can). A handful of sand mixed in with the PVA will provide a rougher texture when the PVA has dried – ensuring better adhesion. PVA can be purchased at any builders merchants or DIY store. The suction test should then be carried out on the PVA surface. If it is still very porous then anther coat of PVA should be applied to seal the wall and make it less porous.
Backing Coat video