The backing coat is responsible for flatness.
The purpose of the backing coat is to achieve a totally flat surface – it doesn’t matter how rough it is as long as it is flat. Average thickness is 12mm.
The skim (finish) coat is responsible for smoothness.
A smooth finish is obtained by applying a skim coat of finish plaster. The skim coat is applied to the backing coat at a thickness of 2-3 mm. It is not designed to flatten, but is literally just a smoothing off coat. The Skim Coat is explained in section 2.
All walls need a backing coat except
Plasterboard walls - the plasterboard acts as the backing coat as it is totally flat. If for some reason the plasterboard is not flat then a backing coat of ‘Bonding Coat’ can be applied.
Previously Plastered Surfaces - for example – when the wallpaper has been removed in old houses the wall behind although flat may be in too poor a condition to apply paint. Therefore a skim coat should be sufficient.