Plastering Coats Explained: Bonding, Hardwall and MultiFinish

If you have had a plastering quote, you may have seen words like Bonding, Hardwall, browning, float coat, MultiFinish or board finish. They all describe different plasters and coats, and picking the right one for the wall matters. This guide explains, in plain words, what each one is, where it goes and why.

RCR Connections Ltd is Birmingham based, covering the whole of the Midlands. Our team has 62 years of combined experience, and we plaster new extensions, old Victorian terraces and everything in between.

Two kinds of coat: backing and finish

Most plastering comes down to two kinds of coat:

• Backing coats (also known as undercoats, float coats or browning) go on first. They are thicker, usually around 8 to 11 mm, and their job is to fill hollows, straighten the wall and give a sound base.
• Finish coats go on last. They are thin, about 2 mm, and give the smooth, flat surface you paint.

A sound old wall may only need a finish coat. A wall that has been hacked off back to brick needs a backing coat first, then a finish coat. Plasterboard needs a finish coat only.

Hardwall: for brick and block

Hardwall is a gypsum backing coat for walls with high suction. Suction means how quickly the wall soaks up water. Common brick and most building blocks are thirsty, so they pull water out of the plaster fast. Hardwall is made to cope with that.

It sets fairly hard and resists knocks well, which is why it is popular for halls, stairs and busy rooms. It usually goes on at about 11 mm on walls, ruled flat and then scratched or keyed so the finish coat has something to grip.

Bonding: for low-suction surfaces

Bonding is a backing coat for surfaces with low suction. These are smooth, dense surfaces that do not soak up much water, such as:

• concrete walls, lintels and ceilings;
• engineering brick;
• some painted or glazed surfaces, once treated with a bonding agent.

On these surfaces a plaster made for brick would not grip well and could come away later. Bonding is made to stick where others would not. It is also used for building up patches over mixed backgrounds, such as a brick wall with a concrete lintel.

MultiFinish: the finish coat

MultiFinish is a gypsum finish plaster that works on most backgrounds: Hardwall, Bonding, sound old plaster and plasterboard. That makes it the most common finish coat we use.

It goes on about 2 mm thick, in two coats. The first thin coat goes on and is flattened. While it is still firm but damp, the second coat goes on over it. Then, as it starts to set, it is trowelled several times until it is smooth and tight. Timing the trowelling is where the skill lies.

There is also a board finish plaster made just for plasterboard. On board we often use MultiFinish anyway, because it gives a good finish on everything.

Plasterboard: boards first, then finish

A lot of modern plastering starts with plasterboard rather than wet backing coats. Boards are screwed to timber or metal studs, or stuck to the wall with dabs of adhesive. The joints are taped, then a finish coat goes over the whole surface.

Boarding is quicker to dry than wet backing coats, which helps in new extensions and in winter. Our plasterboarding and dry lining page explains when we board and when we use wet plaster.

Lime plaster: for old solid walls

Many houses built before the 1920s have solid brick walls and lime plaster. Lime is softer and lets moisture move in and out of the wall. Putting hard gypsum or cement-based plaster on these walls can trap damp, especially low down and on outside-facing walls.

For repairs on old lime plaster, we often match like with like. We look at the wall, the age of the house and any damp before deciding. Our plastering repairs page talks about old houses in more detail.

Why the right coat matters

Using the wrong plaster is one of the main reasons plaster fails. Common faults are:

• Blown plaster, where it loses its grip and sounds hollow. Our blown plaster guide explains this.
• Cracking, from coats that dry too fast, are put on too thick, or do not suit the wall.
• Damp and staining, where gypsum has gone on to a wet or salty wall.
• Soft, powdery finish, from plaster drying too fast on a thirsty wall.

The same thinking applies outside. With render, every coat should be no stronger than the one below, and render should never bridge the damp proof course. You can read about that in our guide to render and the damp proof course.

How we choose the coats for your job

1. We look at what the wall is made of and how sound the old plaster is.
2. We check for damp and any cause that needs fixing first.
3. We pick the backing coat, if one is needed, to suit the wall.
4. We put on a MultiFinish coat, about 2 mm in two coats, for a smooth surface ready to paint.
5. We tell you how to dry the room and roughly when it will be ready for a mist coat.

Drying time depends on the coats. A finish coat on plasterboard dries far faster than a full backing coat and finish on brick. In winter, all of it is slower. See our guide to plastering in cold weather and our page on plastering walls and ceilings.

We also fit K-Rend monocouche and silicone render outside. If you are having work done inside and out, see our K-Rend in Birmingham and Solihull page.

Not sure what your walls need?

We work Monday to Friday 7am to 5pm and Saturday 7am to 1pm, and we are closed on Sundays. RCR Connections Ltd is Birmingham based, covering the whole of the Midlands. You can see the towns we cover on our plastering areas and rendering areas pages, and finished jobs on our projects page.

More guides: Plastering in cold weather · How long plaster takes to dry · Blown plaster