As with all diy projects, the correct preparation of different surfaces for plastering is essential to the success of the finish. Failing to prepare the surface to be plastered can result in numerous problems, including flaking and a "blown" surface.
The first thing you should do is brush down the surface with a stiff-bristled brush to remove any dust, dirt or efflorescent salts. Now you can test the absorbancy of the bricks or blocks by splashing water on them. If the water stays on the surface, the bricks can be considered normal and can be plastered over with only a light dampening. If the water is absorbed quickly into the bricks you will need to soak the bricks before plastering. You can use a large brush for this. A dry, absorbing brick surface will strip moisture from the plaster too quickly and cause it to crack and flake.
High-Absorbency Surfaces
Surfaces such as Aerated Concrete blocks are far too absorbent to be plastered over without some additional preparation. Mix up 1 part bonding agent (available from DIY stores) with 4 parts clean water and paint the mixture onto the surface. When this is completely dry, make another mix of bonding agent and water (this time 3 parts bonding agent and 1 part water) and paint this onto the surface. Leave this to get slightly tacky and then apply your plaster.
Low-Absorbency Surfaces
Most other brick, block or concrete surfaces will also need to be treated before plastering. Mix 1 part bonding agent with 4 parts water and brush onto the area to be plastered. Allow this to dry and then apply a second coat with a mixture of 3 parts bonding agent and 1 part water. Allow this to dry for 24 hours and then apply your plaster.
Non-Absorbent Surfaces
Surfaces like tiles and painted walls are non-absorbent and will require a coat of bonding agent (neat, not diluted) to allow the plaster to key. Once the bonding agent has been applied evenly, start to plaster before it has dried. To give ceramic tiles an even better key, try mixing 2 parts sharps sand, 1 part cement, 1 part bonding agent and 1 part water. Mix this well and apply the "slurry" with a stiff-bristled brush all over the tiles. Leave this to dry for at least 24 hours and then apply your plaster.
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