Many people try to skip over using primer on their projects, mostly as a way of saving money and labor. What they don’t realize in doing this, is that they are taking away from the ultimate quality and beauty of their labors. Regardless of the kind of finish one is using, the right primer helps to make the finish turn out well, giving the best possible results for this all-important final step in any project.
The other mistake that many people make is to use other materials in place of primers. Regardless of what anyone thinks or says, paint and primer are not the same thing; nor can one perform the task of the other. Paint is designed to provide an attractive, smooth finish. The dry time is gauged to allow the finish to flow out, reducing brushstrokes. Primer is heavier bodied, so it can fill the pores in the substrate, sealing it.
There are three main characteristics that most primers have:
- High solids, to fill pores in the substrate
- High adhesion
- Fast drying times, so that the primer doesn’t soak into the substrate too much
None of these characteristics are built into paints or varnishes, regardless of whether we are talking about oil-based or water-based finishes. Therefore, when we try to use paints or varnishes as primers, what we end up with is the finish coat soaking into the substrate and not sealing it properly. Additional coats of finish continue soaking in, until the substrate is finally sealed. Only then does the finish coat start building to form a smooth finish.
The first primer coat to go onto a new project is the most important, as that is what determines how every coat of finish ever applied on top of it will bond. If you don’t use quality primer at that point, there is nothing you can do later to make up for it.
Ultimately, the lack of a primer coat ends up costing more than the amount saved, because primer is usually less expensive than the finish material used. But that’s really only a secondary effect, as the quality and durability of the finish suffer.
2 in 1 Products
A number of paint manufacturers have developed 2 in 1 products which boast being a prime and paint finish in one. While these products are different than normal paint products, they really don’t quite meet up with the quality of using both a primer and a finish paint.
The way manufacturers make these products is to add binders to the paint, which help it to adhere well to a bare substrate. If the paint itself is heavy-bodied enough, it works well as a pore sealer, fulfilling that purpose. But what you’re doing, when using a product of this type, is that you are using the more expensive paint as a primer. You’ll probably need to apply two additional costs of paint, to get a good finish on your project.
Before primer is applied, the wood should be sanded. If the wood is going to be painted, wood putty should be applied in knotholes, cracks, dings and nail holes, allowed to dry and then sanded flush with the surface of the wood. In some cases, a second application of wood putty will be required, as it tends to shrink when drying.
Proper sanding of wood takes multiple stages. If you are working with appearance grade pine lumber, you’ll want to start out with #120 grit sandpaper and end up with #220 grit. For hardwoods, start with the same #120 grit, but only work your way up to #180 grit. A general breakdown of different grits of sandpaper is:
- Very coarse – #16 to #30 grit – for use on very rough unfinished wood
- Coarse – #36 to #50 grit – for use in distressing, rounding and rough areas
- Medium – #60 to #100 grit – for removing rough texture
- Fine to medium fine – #120 to #220 – fine sanding before staining
- Very fine – #240 to #600 grit – polishing and finishing after staining
Please note that sanding before staining and varnishing is different than sanding before painting. Generally speaking, a finer sandpaper is used before staining, than is used before painting. But the finer the sandpaper that is used on wood that is to be stained, the lighter the finish color will be, due to less stain soaking into the wood.
How Sandpaper Works
Sandpaper is an abrasive, so what it’s actually doing to the wood is scratching it. Hence the sawdust that is produced by sanding. As you go to finer and finer grits of sandpaper, smaller and smaller scratches are made, replacing the deeper scratches made by the coarser paper. Eventually, the scratches are so fine, that you can’t see them without extreme magnification.
This same idea is used in polishing anything from furniture to jewelry, even diamonds. At all levels, finer and finer abrasives are used, making smaller and smaller scratches. Eventually, the scratches are so small, that the surface is polished and becomes reflective.
Using a Scraper Instead of Sanding
Another option for smoothing wood, which has been in existence since before sandpaper existed, is to use a scraper. Scrapers are flat pieces of metal, on which a burr is raised along the edge. This burr cuts off an extremely thin shaving, much finer than you can get with a plane. When used by someone who knows how to get the most out of it, a scraper can give a finer finish than sandpaper does.
One of the big advantages of using a scraper over using sandpaper is that sandpaper makes the surface of the wood fuzzy, which is more of a problem on some woods than others. That fuzz can end up in the finish as particles which cause bumps in the paint or varnish. Since a scraper cuts off material, rather than making scratches, there is no possibility of that fuzz.
Applying Primer to Wood
In most cases, primer should be applied with a brush, unless large areas need to be primed, then it can be sprayed. By and large, the only time that a roller should be used for the application of primer is when applying it to drywall. Using a roller to apply primer to wood may not give you a sufficiently thick application of primer.
Primer should be painted on in the direction of the grain. If end grain needs to be primed, do it first, as the end grain will absorb more primer than the rest of the board. This will help you to prevent runs and drips on the board, which might occur if you prime the end grain after priming the rest of the board.
The big question for most people, and the main point of this article, is how many coats of primer to apply. The assumption is usually to apply only one coat of primer; but that is incorrect. Most primers need two coats to fully seal the wood, before applying the finish coat.
Primers are fast-drying. You should check the label for actual dry times, but most primers will dry within 15 to 30 minutes. Don’t apply a second coat of primer, until the first is fully dry. You’re better off waiting too long, rather than applying the second coat too soon. While primers are dry enough for a second coat in 15 to 30 minutes, they are not actually fully dry for about three hours.
For fine finishes, each primer coat should be lightly sanded with fine sandpaper, before the application of the next coat. Then the wood should be wiped down with a damp or tacky cloth, to remove any dust. The main purpose of sanding the primer is to remove any bumps caused by dust or fuzz, which might show through the final finish. Run your hand over the surface, after sanding, to check for any of these that you might have missed.
Applying Primer to Metal
Applying primer to metal is much like applying it to wood, although different primers are used. Most of these are oil-based, although there are some newer water-based primers which will work on metal. You will need two coats of primer for metal, just like you will for wood. It’s extremely difficult to get a full coverage of primer, without the metal showing through, using only one coat.
Primers on metal aren’t intended to seal the pores, as there aren’t any to seal. Rather, they are mostly to provide adhesion. This is especially important for aluminum and rusty metal, both of which are surfaces which are hard to get paint to properly adhere to.
Applying Primer to Drywall
Drywall is different, in that it has a surface that is somewhat sealed from the factory. Therefore, you don’t need to use two coats of primer on it, like you do with wood. One coat is sufficient. That one coat can also be applied with a roller; the only time I would use a roller with primer.
There are a couple of different shellac based primers on the market; most notably Kilz and BIN. These primers are alcohol based, rather than oil or water. That makes them extremely fast-drying, which is important for their intended purpose. They also have a heavy amount of pigmentation. This combination of being alcohol based and having heavy pigmentation makes them excellent for use in covering stains. The primer is able to dry, before the stain can leech into it.
Being alcohol based, these primers can also kill bacteria and fungi, many of which cause some of these stains. However, alcohol is not good at killing mold; so if you have mold that needs to be dealt with, you’re better off using bleach or vinegar on it, before covering it with a shellac and alcohol based, stain-killing primer.
As with other primers, two coats of these products should be used. It is not necessary to apply another primer over them, as the finish coat can be applied directly to these types of stain blockers, just as it can to other primers. These types of primers have excellent adhesion to the substrate and provide a good primer to paint being applied over them.
These products are also useful when wanting to apply a light colored paint over a pre-existing darker paint. Their stain blocking capability will help ensure that the darker colored paint will not show through the new paint. In contrast, if a primer of this type is not used, you may have to apply as many as three finish coats to cover up the existing paint color.
Painting Over Primer
While it is possible to leave a surface primed, without painting over it, most primers do not provide an attractive finish. They generally have a very dull sheen, to encourage bonding of the topcoat. Color may not be uniform, as they are not heavily pigmented and intended to be visible. Nor are primers UV stabilized, so they might be affected by the sun.
Typically two coats of paint should be applied over most primed surfaces. You can get away with one coat, in the case of some very heavy-bodied paints. However, there’s a good chance that you will have pinholes in the paint, where the primer below shows. Applying two coats ensures that you have a full, even coat, providing the best possible finish.
For Stained and Varnished Wood
The same sorts of primers aren’t used for stained and varnished woods, that are used for painted wood. Stain does not function as a primer for the varnish, in these cases. A product called “sanding sealer” is used as a primer for varnish, performing the same purposes as primer does for paint.
As mentioned earlier, the more you sand your wood and the finer a sandpaper you use, the lighter your stained finish will appear. Varnish, and to a lesser extent sanding sealer, will also affect the final color of your stained wood. You should always do a test of the finish on a scrap of the same wood you used in the project, to verify how your finish is going to turn out, before finishing the project.
There is another product which may be used in the process of staining and varnishing wood; that’s “paste wood filler.” This is a product designed for filling the pores of open-pore woods, such as oak and mahogany. Filling the pores, before applying the varnish, provides a much smoother final finish. If paste wood filler is not used, projects made with these woods, and other open pore woods, will never provide a glass-smooth finish.
Paste wood filler can be tinted, allowing you to create some interesting effects or to change the grain color of the wood you are using. Standard tints used for paint are used to tint paste wood filler as well. The filler can be applied either before or after staining, as you prefer. You may want to try making samples both ways, as the paste wood filler and the stain will affect each other’s coloration.
Holes to be filled on stained and varnished pieces should not be filled before staining. They should not be filled until after the sanding sealer is applied, has dried and has been fine sanded. Applying wood filler to the project to fill nail holes before that point in time will end up causing spots where the stain and/or varnish are uneven, giving the appearance of spots in the wood around the nail holes.