The simple answer would be use a primer when painting on a surface for the first time and an undercoat when the surface has been previously painted. However, it is never that black and white.

Undercoats, often used on wood, are a little denser than primers and are intended to produce a smooth, even surface for the topcoat. In tandem with the topcoat, they can produce a really good looking finish.

Primers primarily heighten the bonding process, seal and well, prime, for the topcoat! They can offer extra ingredients such as additional UV protection, waterproofing, or extra penetration. Primers promote surface wetting and bonding with formulas specific to the topcoat in question. This is why you are always advised to use the topcoat manufacturer’s recommended primer.