Complete Guide to Plaster

Plastering is a DIY task that can often turn into a nightmare. Plastering is a skill that can only really be mastered with a lot of practice. This is especially true with “professional” plaster. This is usually more difficult to mix correctly, sets quicker and unless you really know what you are doing, can drag off the wall easily. DIY plaster has been formulated to solve many of these problems, but it is still important to understand the basics of plastering.

Types of Plaster

There are several different types of Plaster available, and choosing the right type for the job at hand is important to the durability and finish of the plastered area.

Professional Plaster

Gypsum Plaster is the choice of the professionals and is usually applied in two coats. The first coat, the Undercoat Plaster, is applied to about 10mm thick, and scored to provide a key for the second layer. The second player, or skim, should be applied to around 3mm thick in a single layer. If working on a particularly absorbent surface, a HSB (High Suction Background) undercoat plaster might be used. Specially formulated Finish plaster can also be bought for skimming plasterboards.

DIY Plaster

As previously mentioned, DIY plaster has been made to overcome some of the difficulties of pro plaster. DIY plaster comes in two basic forms; Repair and Finish, and is readily available in pre-mixed tubs or ready-mix bags.

– Repair plaster, also sometimes called one-coat plaster, is very easy to apply and can be used in thicknesses of up to 50mm. Repair plaster is especially good for filling wide and deep holes in walls such as those left when a pipe has been removed. Repair plaster needs to be “keyed” to give the finish plaster something to adhere to.

– Finish plaster is used over repair plaster to give a smooth finish for painting, papering or tiling. It can also be used directly over plasterboard or simply to smooth existing plaster. Finish plaster should only need to be around 3mm thick.

Preparing Masonry For Plaster

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 absorbency 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.

Plastering

Make sure you have the right tools before you start. You will need: Plasterers Trowel, Hawk (small, flat board with a handle) and a Straight Edge. Load the hawk with the mixed plaster and carry it to the wall. Scrape a small amount onto the trowel and spread it onto the wall in a sweeping arc. Never hold the trowel flat to the wall, keep the blade at a slight angle and let the plaster squeeze through the gap between it and the wall.

Repeat this until the area you are plastering is covered. When the plaster has dried out slightly, you can use the trowel to smooth it out. If you are plastering a small area, fill it with the repair plaster in the same way and then use the straight edge to level it off. Hold the straight edge against the wall, and move it over the plaster in a side-to-side motion.

Finish plaster is applied in basically the same way. Just ensure the plaster is not applied too thickly. Smooth off the finishing plaster with the trowel held flat. As with the repair plaster, this is best done when the plaster has dried slightly. Plasterboard can be skimmed with finish plaster, or the joints can simply be covered with Joint Tape and the nail holes filled, before papering or tiling over it. If you are planning to paint the wall, a skim will probably give a better finish.

Plastering External Corners

To plaster external corners, such as on a chimney breast, you will need to use metal beading made specifically for this task. This L-shaped strip of metal is made of a solid edge with mesh sides. You need to attach the beading to the corner using small blobs of plaster. The mesh sides will then give the plaster you apply to the corner something to adhere to. You can read our full guide to Plastering External Corners for more information.