There are a variety of different paint primer products for use around your home. Some primer paint is used over drywall and is a latex based paint. Other primers are specifically for metals or plastics and have their own special properties not suitable for wood. On any outside painting project you always want to make sure that the product you are using is rated for exterior use. Never think that you can get away with using interior grade products whether it be paints, stains or primers outside even if you are only completing a small project.
In addition to being exterior grade you also have a variety of wood primers for different applications. There are some that are specifically rated for mold killing or resistance. You should not use these products in every application even though you might think that it is a good idea to protect the wood and paint from mold. These products are specifically made for areas that you know or suspect will have mold problems. You are also likely to pay a premium of about 30% for mold fighting primer so they are best suited for specific areas and applications.
Choosing Between Oil Based, Latex Or Alcohol Primer Paint
The next consideration is the base that the primer is made of but it is not an easy choice to make. The choice really depends on if the surface is new or if you know what product was previously used on it. If you know that Latex was used then you should use latex primer and if you know oil based stains or paints were used on the surface then you should use Oil Based Primer. Many people will consider the age of the home and say that if it was painted before 1970 then there is a good chance that it was painted with an oil based paint and this is normally correct. After the 1970s then you are likely to have a latex product but because Oil Based Paints, Stains and Primers have never become completely unavailable there really isn’t a best method for knowing what product it was painted with.
Because latex paint is a water based product you never want to use Latex Primer or Paint over any surface that has had a Oil Primer, Oil Paint or Oil Stain.
Testing For Water Based Vs Oil Based Paint
If you just don’t know which paint was used either latex or oil based then you can perform a simple test. Get a clean white rag and apply a generous amount of rubbing alcohol and rub the surface of the paint. If the towel has paint transferred to it then the paint is latex. If there is no paint transfer then it is Oil Based Paint.
Unfortunately if the paint is heavily oxidized then the oxidation powdered paint will come off on the rag and you can get a false positive so this test is best if the paint is stable or glossy and not powdery on the surface. Make sure you remove all of that oxidation before doing your test.
Using The Right Primer When Painting New Wood vs Previously Painted Wood
If you are painting new wood and you are not just painting a patched area then you should probably use a latex based primer and paint. I say probably because the technology of latex paints is really the primary concern of paint companies today and oil based paints although good are not seeing the advancements that latex is. Also latex paint is much easier to work with and the cleanup only requires regular soap and water. If someone is suggesting that you use Oil Based Exterior Paints on New Wood then you should ask why they believe they are a better application.
However one problem that might be seen with Latex Primers and Paints is their ability to hold out the resins that re in the wood. This is true with Pine primarily where large knots in the wood will seep through the primer. In this case you should first apply an alcohol based primer before your latex primer and this will resolve this problem. Alcohol based primers dry very quickly allowing you to continue work quickly and are not needed on the entire surface normally.
If you are painting previously painted or stained surfaces then you should test the paint that is existing and use the product that matches what you have. Your other option might be sanding the wood to a level that is below where the oil based product has absorbed into the wood. This is possible when you are staining something like a deck that has 5/4″ wood decking and rails and you will be sanding it back significantly to remove surface damage. You do risk some pealing but it is much less likely to happen if you remove all of the stain that is visible. On moldings or other detailed surfaces you do not have this option of sanding enough to remove the product from the wood surface. In this case you should continue using Oil Based Primer, Stain or Paint if you know the surface has previously been painted with that type of product.
Final Note
Never use latex paint directly over any surface that has been previously painted with oil based products or you will likely have pealing or early failure and then you will be blaming the latex paint as not being as good. It is not the problem of it not being as good it is the problem of oil and water not mixing.
When in doubt you could always go with Oil Based Paints but it is really not a good choice to switch products like this. Use Oil based over Oil Based or Oil over Latex but never Latex over Oil and always try to use what was previously used.
If you end up with pealing then you can use a stripper to remove what you just applied and then repaint.