I was reworking the undercoats and the primer pages on our site, and realised it might be helpful to clarify what the differences are between an undercoat and a primer.
A primer allows a surface to take paint, to act as a layer to join the top coat (IE Auro 555 emulsion) with the substrate, (IE bare plaster). A primer helps control moisture absorption, and control different surfaces (IE your bare plaster and your filler) that also may have different absorbencies.
An undercoat is a paint that is designed to provide a base for even colour, so traditionally you would use an undercoat over a primer, and under your top coat. It evens out the patches, and allows for a consistent colour finish for your top coat emulsion. IE bright blue paint on pink plaster would end up looking darker than had it been painted over a nice bright white, even and consistent undercoat.
Technically, one could argue that undercoat 'is' paint, so therefore only really adheres to paintable surfaces.
An undercoat is still usually always still a primer, but a primer is not an undercoat.
So... in reality.
If you have bare plaster walls, you need something as a primer (or you could say mist coat).
Do I need a primer or undercoat for bare plaster?
Auro paints: For a normal top coat like Auro 555 emulsion, you can just use a water thinned coat of the white Auro 524 to act as a primer and undercoat. 10% water, 90% Auro 524 natural emulsion.
Graphenstone Paints: If you are using Graphenstone Grafclean emulsion as your main colour over bare plaster, best bet is to use a coat of the economy trade white from Graphenstone, the Graphenstone Nevada Ultra.
Do I need a primer or undercoat if my wall is already painted?
Auro paints: For a normal top coat like Auro 555 emulsion, you can just use a water thinned coat of the white Auro 524 to act as an undercoat. 10% water, 90% Auro 524 natural emulsion. However, if you are painting a darker over a lighter colour you generally don't need more than 2x coats of the Auro 555.
Graphenstone Paints: If you are using Graphenstone Grafclean emulsion as your main colour over existing paint, unless you are painting a very light colour over dark (if so then use a coat of the economy trade white from Graphenstone, Nevada Ultra for best colour) you don't need anything other then 2x coats of the Graphenstone Grafclean.
I hope that helps!
