You're getting ready to paint your house and you aren't certain what type of paint to buy. If the old paint is latex, another coat of the same is in order. If the old paint is oil base, however, an oil primer is an absolute necessity, followed by a finish coat in oil or latex.
The important question is which type of paint currently covers your home. Here's an easy way to find out.
First, scrub a small sample area with a strong solution of common household detergent.
Rinse the area with plenty of fresh water and then towel-dry it.