When painting and in order to get the best results form any painting exercise, it’s often a very good idea to consider using a primer or sealer before you apply the paint. The question which people find is the most difficult is to be able to decide when to use a primer or sealer and then or course to be able to determine the type of product to apply are mysteries to not only to do-it-yourself homeowners but often for many professionals.
The Advantages of Primers
Primers are specially formulated coatings that perform several valuable functions:
First, they help the paint adhere better to the surface that’s being painted. They do this by making the surface more uniform and acceptable for the application of the top coat, so that when the paint is applied, it can get a better “grip” than it would on a plain simple bare surface.

Second, primers help to supply any finished painting exercise a much more uniform appearance both in terms of colour and sheen, which of course makes it much more attractive. This is especially true when for example the surface being painted is porous or uneven in porosity.

Third, some primers are very good to help prevent stains from appearing through the paint from the underlying surface below ruining the appearance of the finished coat. This is especially important with any latex finish coats, which otherwise can be vulnerable to some stain coming through from the underlying surface.
When buying any primer or sealer it is obviously very important to understand whether or not stain blocking is one of its capabilities and, if so, what types of stains will be blocked.

Sealers Enhance Uniformity
Closely linked with primers are sealers. While primers are often pigmented, sealers usually are not.

The role of a sealer can be to seal a porous surface like weathered concrete or stucco, so that a final finishing coat can develop a uniform sheen or gloss. Sealers also help protect the finish coat on masonry from any efflorescence and alkalinity. Some sealers are used on stone masonry to help to seal out moisture, in which case the masonry is often left unpainted.
When to Use a Primer or Sealer

To achieve the best results and a first class result for any use of paint either at Home or at the Office , I would always recommend a primer or sealer in the following circumstances:-
When new unprimed wood, new drywall or any other surface that has never been painted before is to be painted for the first time. Prior to priming, it is a good idea to seal knots with a specific Primer Sealer for that purpose and also prior to priming, it would also be a good idea to putty any nail holes, cracks and blemishes and these could be spot primed with specific undercoat paint.
When one is repainting a surface that has been stripped or is worn down to the original surface material.
Just like paints and other coatings, primers and sealers always perform best when the undersurface is properly prepared. Regardless of the application, the surface to be primed should always be clean and free of all dust, dirt, grime, flaking or loose paint and indeed any other contaminants before the primer is applied.