There are a few things to consider when buying exterior paint. The type of paint (latex or oil based), the surface to be painted, and the outside temperature are three factors that you need to keep in mind.
There are two types of exterior paint available. Latex paint is a water-based product that is relatively easy to work with. It dries to the touch in less than an hour and cleans up with soap and water. Latex paint is the preferred one for exterior use because it "breathes." This means that the paint expands when the temperature goes up and contracts during colder conditions. As a result, this product is very durable.
Oil-based paint is also available for exterior use. This product takes approximately four hours to dry, and you will need to consider that when you are buying exterior paint. It covers well and you will need to use turpentine to clean your brushes and rollers after using it. Unlike latex exterior paint, oil-based paint products don't expand and contract as the weather warms and cools.
Latex paint has an interesting property in that it cannot be applied over a surface previously painted with an oil-based paint. This is a factor to consider when buying exterior paint, since you may not know what was used to paint the surface already. Applying latex paint on a surface previously painted with oil based paint may cause the latex paint to bubble up as it dries. You would need to sand the entire surface smooth before repainting it. The safest choice if you are looking at a previously-painted surface and you don't have this important information is to use an oil paint.
Before you start painting, you will need to prepare the surface, which needs to be clean and dry. Any loose or peeling paint must be sanded down until smooth. Holes or cracks in the wood need to be filled before painting. Remember to add the necessary supplies to prepare the surface to your budget when you are thinking about what to consider when buying exterior paint.
The outside temperature is another thing to consider when buying exterior paint. Check the label on the paint you are considering using to find out what temperature is recommended for use of the product. If the label indicates that you can use it if the temperature is 50 degrees or higher, this includes the overnight reading as well as the daily high. The weather must be fair, with no rain in the forecast for at least 24 hours after you finish painting.