Primer is used on exteriors for different reasons. Sometimes you need to prime for adhesion to a hard to grip to surface like glossy paint. Sometimes you need to prime to prevent the substrate from rusting. Stain blockers are used to prevent tanin bleed from migrating into the finish paint film. Primers are used on raw wood to aid in penetration and anchoring and to seal the porosity of the substrate.
Primers are used on the exterior as:
- Stain blockers
- Raw wood primers
- Bonding primers
- Knot sealer
Let’s look at the primers and consider the reason for their use.
Oil or latex
There are two types of basis primer vehicles: latex and oil. Actually there is a third, shellac which is limited to spot priming only on the exterior. Most DIY’ers like to stay away from oil based primers and paints, but they have some advantages over latex. Oil primers are much better at penetration into raw wood and dusty or chalky surfaces. Latex primers on the other hand, are more breathable and more flexible than oil, especially over time.
My favorite exterior house painting primers are: 123 Bullseye, Sherwin Williams A-100 alkyd (oil) primer, Coverstain alkyd primer, Bullseye shellac (spot primer for knots only), and Emulsbond stir in primer additive for latex paint.
Coverstain
Coverstain is a quick dry alkyd alternative for spot priming and for times when you just can’t wait two days for the primer to dry. Coverstain is a very good stain blocker for a variety of stains including tannin stains, as well as for other stains. This primer has excellent adhesion , and good flexibility (for an oil). Coverstain is a bonding primer and will adhere to many difficult to grip to substrates, including glossy paint.
Latex Primer
123 Bullseye
123 Bullseye is a universal primer used for stain blocking, bonding and sealing. It is not as good a stain blocker as Coverstain, nor is the adhesion as good as a bonding primer. 123 Bullesye is a good flexible and breathable primer which makes it very peel resistant. Good for use as a first coat total surface primer (tint to match finish paint). Great for aluminum siding, vinyl, wood and a variety of substrates. Quick dry latex formula means you don’t have to wait for tomorrow to paint.
Shellac primer
Both clear amber shellac and white pigmented shellac are used to seal knots in wood. Knots are very tough to block and shellac is the only thing that works on some knots. Use shellac only as a spot primer on the exterior. It lacks flexibility and breath-ability so it cannot be used as a whole surface primer. I like to overcoat shellac with a coat of oil (Coverstain) primer, before finish painting. See Knot Sealers
Specialty primers
Emulsabond is one of my favorite “primers”. Actually, it is a mix in additive that will convert you finish paint into a primer / finish. EB is a penetrating oil that is used for chalky and dusty surfaces. Made for latex paints (only, use Penetrol with oil paints), it works very well for binding chalky paint. It is not a bonding primer additive for glossy surfaces, it binds chalk and dusty surfaces only. I have used this for years. Use it in the first coat of paint as a perfectly matched primer/first coat. Used with skill, it can be used with flat paint as a one coat primer / finish paint mixture. Read the label for more specific instruction for it’s use. For exterior use only.
Self priming paints
Paint manufacturer’s are not trying to make some of their paints both a primer and a finish paint. They do this by using multiple size particles in their paints. The very fine particles which are able to penetrate into pores of raw wood and other substrates. The medium and larger particle sizes are non-penetrating and add to film build on the surface on the substrate.
Read the label
Most of these “self priming” paints are only self priming for one or two purposes. They don’t cover the full spectrum of priming needs. Duration, Sherwin Williams excellent self priming exterior paint, is only self priming over raw wood. It is not self priming as a bonding primer over glossy surfaces, nor is it a stain blocking primer. You will need to prime underneath it for bleeding stains or for adhesion to glossy paint. Zinsser on the other hand makes there Pema White exterior paint as a self priming finish paint over glossy surfaces as well as bare wood. Read the label for each of these paints and see what they are self priming for.
Oil Primer
Use oil when you are painting redwood , cedar or other woods which have extractives that are prone to bleeding into the finish paint. Oil has better adhesion to less than perfectly cleaned and prepared substrates.
Sherwin Williams A-100 oil primer
A-100 primer is a general first coat primer, used on substrates where the whole surface will be primed. Generally, I will use this as a first coat, and tint it to match the finish paint color. This is a “medium oil” primer that will take 24 - 48 hours to dry (one of the downsides). It has excellent penetration into raw wood, and mild chalk. A-100 is a good stain blocker for bleeding tannin stains.
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