Choosing and Using Paint Primer
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If you want to waste your time when painting then don't prime bare surfaces. The quickest way to have a paint job fail is to fail to prime new materials. In fact, priming is one of the most important things you can do to ensure a lasting paint job, particularly outside.
Surfaces are painted to protect them from the effects of the outside world. Rain, pollution, wind and heating systems and human contact inside, can degrade a surface making it dirty and stained or even decayed. To prevent this degradation we paint or otherwise apply a finish coat to almost every material we use.
Paint and Primer All–in–One
There are a couple of paint manufacturers that are now making paint that is both primer and finish coat in one. Is this a good idea? The short answer is No. The whole idea of primer is to bond with the substrate surface, whether it's drywall, wood, metal or any other material, and create a foundation for the finish coat.
You can't expect a paint to do everything that paint should do. For instance, you can't expect a flat paint to resist hand smudges and stains like eggshell and semi–gloss do. To expect that a paint can penetrate and bond to a surface and give you a uniform, smooth finish, all in one coat, is just as overly optimistic.
If you want your paint job to last you should always prime bare surfaces, and with a real primer. Even using a so called paint–primer–in–one, and doing two coats, will not be the same as priming with an appropriate primer and following with a good finish coater.
Which is the Best Primer?
So what is the best primer to use? It depends on the surface and the conditions it is under, i.e. inside or outside. For the average homeowner/do-it-yourselfer there are really only a few variables to consider. First you will be dealing with wood surfaces both inside and out. Secondly, drywall and plaster surfaces inside will come into play and sometimes vinyl or aluminum may need attention when it comes to exterior siding. Each of these surfaces should be treated properly to ensure a lasting finish.
Wood, whether inside or out should be primed with an alkyd or oil primer. Latex is not a good choice for priming wood because of the moisture issue. Wood will absorb and expel moisture through its pores constantly as the atmosphere changes. This is particularly true outside where rain and humidity, sun and wind affect the moisture content in the wood. If you use a water-based primer the coating will lose it's bond with the wood as the moisture moves in and out, causing the primer to peel off and take the finish paint coat with it.
For drywall an interior, latex primer is a very good choice if you have a lot of drywall to prime. However, if you are only priming a drywall patch or other small space you don't need to buy a separate product to do the job, a general purpose latex primer or the flat latex paint you are using to touch up the patch will do just fine. Plaster can be primed with latex or oil primer but oil and/or alkyd will do a better job of blocking stains that will often be necessary on old plaster. Don't use oil or alkyd primer on bare drywall, it will cause the paper fibers to bristle, creating a rough texture on the surface that's impossible to completely remove.
While latex is the best primer for bare drywall it's not the best choice when drywall or plaster has been wet. Latex will allow water stains to bleed right through and will not block other, residual contaminants that the water leaves behind. This can cause stains and bonding problems with repair compounds and paint coatings. Therefore a shellac–based primer/sealer is required to prepare a water damaged surface for repair and painting.
Working with Oil Primers
Working with shellac, alkyd or oil–based paints can get very messy and the risk of ruining surrounding surfaces is high. There's also the issue of cleaning tools and other surfaces after the job is done. In addition, some jurisdictions have strict rules about using these toxic paints even banning them in some cases. All this makes it tempting to forget about using oil altogether. This is not a good idea unless you want to be doing your paint job over again in a matter of a few months or sooner.
To minimize cleanup and make oil paint easier to deal with try these tips. First try to do all the necessary work at one time. This will eliminate the need to cleanup more than once. If that's not possible use a plastic trash bag and seal the wet tools inside while waiting to continue working. A paint tray and roller can be inserted into a large trash bag and the opening tied shut for a day or more before it starts to dry. Keeping the bag in a cool place with little air circulation will also help to slow drying. Try putting the whole thing in a large cooler or other air tight container.
Use a dedicated brush to apply oil primers and leave the brush in the can when you're done. It will not need cleaning and will be there the next time you need to prime. Some brush handles will be short enough to fit nicely into the average paint can. If not, the handle can be cut to a length that will work. When you need to get the brush out of the can, do it slowly, wiping the excess paint off with a rag and scraping it off on the edge of the paint can. Then clean the handle with a little solvent before beginning work.
For final cleanup wipe all leftover paint from the roller tray into the paint can and let the remainder dry on the tray. Hang the tray somewhere to dry so it can be used again later. If you must clean your brush, let it stand in a bucket of solvent for several days before trying to clean it completely. Then use a small amount of solvent in a coffee can or other small container and rinse the brush by bouncing it on the bottom several times before squeezing out the solvent. Pour the used solvent into a sealable container and rinse the brush again in the same way. When you're done, close the container with the used solvent and let it stand undisturbed for several weeks. The paint will settle to the bottom of the container and the solvent can be poured off and used again.