Role of Primer

• Primer basically acts a safety net for painting. Applying a primer before painting any surface helps to ensure the final coats do not soak through to the surface below and thereby leave an uneven finish. Primer also provides a grip for the finishing coat because paint does not bind to every possible surface.
  
  

Plaster

• Some plastering materials contain a chemical compound called "alkali." This compound remains "hot" for up to three months, which is why an oil-based paint cannot be used over newly applied plaster. The alkali in new plaster will attack the oil-based paint. Applying an alkali-resistant primer can shorten the wait to apply an oil-based paint over new plaster. Other plasters are alkali-free, but use of an alkyd primer is still recommended.
  
  

Curing Plaster

• Primer should only be applied to new plaster once it has completely dried, which can take several weeks. The process of drying is called "curing." New plaster can be tested for curing by dragging a finger across, and if there is a squeak, there is still moisture in the plaster and it is not ready for priming.
  
  

Wet Plaster Warning

• Applying primer to plaster that is not allowed to completely dry will result in an unsealed surface. The surface might be entirely unsealed, or partially unsealed. Either way, unpleasant results are likely, at best an uneven finish and at worst, bond loss and delamination (paint flaking). Similar results will occur if a polyvinyl acetate (PVA) based primer is used over lime gauging or lime-containing plasters.