How to select a Primer or Sealer
Once you have decided that you need a Primer or a Sealer to get the best results from a painting project, it is important to be able to carefully choose the best type of product for the application.
As with all 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 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.
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.
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 soon 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, then it might be best to use an oil-based primer. If the old paint is in sound condition and is still adhering well, priming can be very beneficial, but is not always necessary.
Stucco and Other Masonry.
On new masonry or old stone surfaces that are , a very good latex masonry sealer or primer would be tbest. Where you are repainting use a sealer only where all the old paint has been carefully 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.
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.
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