What is the difference between bonding, hardwall and multi-finish plaster?

The names are trade names as much as material descriptions, which is why the aisle is confusing. The useful split is simple: some plasters are there to build depth and give a flat base, and one is there to give a smooth paintable surface. Everything else follows from which job you are doing and what the background is.

Step by step

1. Decide whether you are building depth or finishing a surface.A wall stripped back to brick needs depth. A plasterboard wall or an existing sound backing coat needs finishing. They are different products.
2. Use hardwall on absorbent masonry.Brick and most blockwork. It is a gypsum backing plaster with good initial suction resistance and it builds a solid, flat base to a decent thickness in one application.
3. Use bonding on low-suction or awkward backgrounds.Smooth dense concrete, engineering brick, a background treated with a grit bonding agent, or a patch that has to build up considerable depth. It is more forgiving about what it sticks to.
4. Use multi-finish over a backing coat or plasterboard.A fine gypsum finish applied in a couple of thin passes and then trowelled to a smooth surface. It is the coat that gives a wall its paintable face.
5. Know that board finish exists as well.A finish plaster formulated for the low suction of plasterboard. Multi-finish covers both in practice, which is why it is the one most people buy.
6. Match thickness to product.Backing coats go on at a substantial thickness in one or two passes; finish plaster goes on at a couple of millimetres. Using a finish coat to build depth causes shrinkage cracking and a weak surface.
7. Respect the working time.Each product has a setting time and it varies with suction and temperature. Mixing more than you can apply and rule off within that time is the standard beginner's mistake.
8. Never retemper plaster that has started to go off.Adding water to reactivate stiffening plaster breaks the set that has already begun. The result is weak, dusty, unreliable plaster that may never gain strength.
9. Mix powder into water, not water into powder.Clean water in the bucket first, then add plaster. It mixes far more easily and gives fewer lumps.

Tips

• Lime plaster is a different family and belongs on solid-walled older buildings that need to breathe — gypsum plaster on a damp solid wall traps moisture and fails.
• Buy fresh. Plaster has a shelf life and a bag that has absorbed moisture in a damp garage goes off fast, lumpy and unreliably.
• Everything must be clean — bucket, mixer, trowel. Set plaster residue in the bucket dramatically accelerates the next mix.
• Never wash plaster down a drain. It sets in the trap. Let it go off in a bucket and bin the solid.

Common mistakes

• Building up depth with multi-finish — Finish plaster is formulated as a thin skin. Applied thick it shrinks, cracks in an irregular map pattern, and often stays soft in the middle because the set is uneven through the thickness. Depth is what backing coats are for.
• Using hardwall on a smooth, non-absorbent background — It relies partly on suction to grab. On dense concrete or a painted wall there is nothing to grab, so it slides and debonds. That situation calls for a grit bonding primer and bonding coat instead.
• Adding water to plaster that is stiffening — The set is a chemical reaction that has already started. Retempering breaks the crystal structure that has formed, and the plaster never regains full strength — it ends up dusty, soft and prone to failure.

Questions people ask