As with paints, there are two broad classifications of primers and sealers: latex or water-based products, and alkyd or oil-based products. Both types can be used for either interior or exterior use. In addition there are also shellac-based primers that have alcohol as their thinner.
In the majority of cases, quality latex primers and sealers perform as well as oil-based products, if not better. However, on severely staining wood and on heavily chalked surfaces, oil-based primers can provide far better adhesion and the ability to block staining.
Exterior Applications
The requirement of using a primer or sealer – and the type of product that should be used obviously can and does vary for each and every painting project.
Here are some general guidelines for common applications:
For brand new Unpainted Wood
If the wood is not severely stained then you could use either a quality acrylic latex or an oil-based exterior wood primer. In the case of severely stained an oil-based stain-blocking primer would be a better choice. It is always a good idea to prime and paint bare wood within say 10 to 14 days in order to keep the wood fibres from deteriorating and reducing adhesion to the primer. Many modern paints do need an undercoat in that a first coat will act as the undercoat and the second coat as the final finish.
Weathered and Unpainted Wood
Here you can use either a quality latex or oil-based primer. It is very important that you clean and sand the wood thoroughly before priming because any deteriorated wood fibres must be removed, or adhesion of the primer will not be so effective. It is best practice to apply the primer as is practical after surface preparation.
Previously Painted Wood
All loose or flaking paint should be scraped off as required and rough edges feather-sanded. Any bare spots should to be sanded thoroughly and dusted off. In addition, as much chalk as possible should be removed before priming. If the old paint is very chalky, and all the chalk cannot be removed, recommend an oil-based primer. If the old paint is in sound condition and is still adhering well, priming can be beneficial, but is not necessary.
Stucco and Other Masonry
On new masonry, or older surfaces that are very porous, a good latex masonry sealer or primer would be the order of the day. Where you are repainting use a sealer only where the all old paint has been removed during surface preparation.
Aluminum or Galvanized Iron
Clean the surface using a non-metallic scouring pad or steel wool (be sure all steel particles are washed off). Then apply a corrosion-inhibitive metal primer to all exposed bare metal.
Ferrous Metals
Remove any rust by wire brushing. Clean and gently rinse off and allow to dry before applying either a latex or oil-based rust-inhibitive primer. I would suggest two coats of primer will provide added protection against future rusting. Rust can come through at a later stage if not properly treated at day one and ruin an otherwise perfect job.
Interior Applications
Usually an interior primer is designed for a very specific application. These can be in both latex and oil-based formulations, so you have a choice of products to choose from. Please however bear in mind that latex products are much lower in smell which is a very large bonus when considering any indoor projects. Particularly if you are living in the property at the time.
Freshly Plastered Walls using English Plaster or Dry Lining
Here a paint that will breathe would be best something like Dulux Trade Fast Matt springs to mind. Using this paint you can paint fresh plaster quicker. It is suitable for dry lining and is ideal for critical lighting. Dulux Trade Fast Matt is a great paint for fresh plaster.
Warning
Special consideration should be given to any surface preparation of pre 1960’s paint surfaces over wood or metal as they may contain harmful lead.