Primers can be solvent- or water-based. Outdoors, use the former (plus a solventbased undercoat); indoors, you can use either, but water-based primer dries more quickly. If in any doubt as to which primer goes with which undercoat, consult your local supplier. Other primers include:
• stabilising primer, for hardboard which is very porous
• wood primer, for wood that’s been treated with a preservative, and for hardwoods.