Come spring, do you join the army of DIYers who march down the aisles of the paint store trying to find that magical paint that’ll last forever? Spring painting chores pop up like dandelions — if it’s not faded or flaking siding, it’s a peeling porch floor. Or the walls in the kid’s room look shabby and worn from washing off marker stains. So we line up at the paint store hoping for a permanent fix.
Unfortunately, there’s no magic formula. No one paint will do it all. So you can expect any paint store clerk worth his or her dropcloth to open the conversation by asking, “What do you want to use the paint for?”
Paint is a specialty product. The economical wall paint that looks great inside won’t survive long under the rigors of the weather outside. And exterior paints, durable as they are, can’t withstand the wear of foot traffic if spread on the floor. The can you pull off the shelf performs best only when you use it for its intended purpose.
In this article, you’ll learn the various requirements of household painting, and then understand how the special paint formulations do the job.
Paints are specially formulated to resist adverse conditions like rain, sunlight, and foot traffic.
What’s in paint
Paint formulators, the folks who mix the paint at the factory. size up the conditions a paint batch must withstand (Fig. A above) and then juggle combinations of three ingredients — solvents, binders, and pigments—to come up with the best formula (Fig B below).
Different types of paint contain various proportions at solvent, pigment, and binder.
What are paint solvents?
They are paint components that make the paint go on smoothly, soak the surface and get a good grip and cover the previous color.
The two most common solvents are paint thinner (for oil-based paints) and water (for latex, or water-based, paints), though some types of paint use solvents like alcohol and lacquer thinner.
When the solvent evaporates, it leaves the binder and pigment behind in a smooth coat (Fig. C). Solvents are cheap.
To form a film, the solvent first evaporates, then the binder cures (hardens) and entraps the pigment.
What are paint binders?
Binders, on the other hand, are expensive.
They’re the glue that forms a durable film, entraps the pigment, and sticks to the surface. Binders are mostly resins like acrylic, alkyd, polyurethane, vinyl, and epoxy, and oils like linseed, soybean, and tung.
You’re probably familiar with some of these names from trips to the paint store. They’re the key ingredients that determine the character of paint.
As a solvent dries, the binders undergo a chemical reaction called “curing,” which lasts from several hours to several days, and produces the tough but flexible paint film.
Pigments, of course, are finely ground particles that color the paint. They also affect its luster (gloss, semi-gloss, flat), extend the life of the film, and “fill” the paint so it covers more surface area.
Paints often contain several pigments, the more expensive, like titanium dioxide (white), for strong color, and cheaper ones like clay and silica (also white). to fill and extend the paint.
Choosing the right paint for the job
Exterior paint has to be tough to protect wood siding from the weather.
It has to stop driving rain, which can penetrate and cause rot, and block sunlight, which can break down the wood fiber. Yet it also must remain flexible enough to stretch as the wood expands and contracts with wide temperature and humidity fluctuations.
And there’s more. It must resist dirt, mildew, air pollution, and casual scrapes from ladders, tree branches, and misguided frisbees. All this without fading.
These are about the toughest demands you can make on a paint film, so paint formulators use premium resins and pigments in high-quality exterior paints (see Fig B).
How can I choose the best exterior paint?
A top-quality formula will use 100 percent acrylic resins for the binder in a water-based (latex) paint.
Specially formulated alkyd resins in oil-based paints work almost as well, but fewer painters use oil-based paints now.
Air-quality laws, which require reduced-levels of solvents (called “volatile organic compounds”), caused them to practically disappear from the shelves in some states.
Water-based or oil-based paints?
Water-based and oil-based paints each have advantages.
Oil-based alkyds brush on more smoothly than water-based acrylics and they dry harder, but acrylic films are more flexible and they “breathe,” that is, they allow water vapor to pass through so damp wood can dry out. That’s a major advantage because it helps keep your paint from peeling. And acrylics don’t break down as easily under intense sunlight.
But oil-based alkyd primers still have some advantages, because bare wood absorbs oil-base solvents (like paint thinner) better than water.
A high-quality primer contains more binder and less pigment, like the gloss in Fig. B, so it adheres well and fills the pores in the bare wood. The topcoat then bonds to the primer and forms a strong, even film.
Many painters apply an oil-based primer to bare wood, then cover it with a water-based acrylic topcoat to form the protective film.
Can I put water-based paint over oil-based primer?
Yes, despite rumors to the contrary, you can put water-based paints over oil-based, but not oil-based over water-based for outdoor use.
Oil-based alkyds dry harder and don’t adhere well to the more flexible water-based acrylics when they expand and contract due to temperature and moisture changes. (indoors, either type will adhere to the other.)
Check with your paint dealer if you have any doubts.
You can use the same binder-rich paints for trim as you do for exterior siding. Both must withstand weathering, but many people prefer to paint their trim with a semi-gloss or gloss luster rather than the flat you’d normally choose for siding (Fig. B).
Higher gloss makes the color brighter and, more important, in the case of oil-based alkyds, it doesn’t chalk as easily. (See below for more information on Chalking.)
Paint companies and dealers often refer to gloss and semi-gloss paints as “enamels,” which loosely reflects their increased binder content.
As you might expect, the higher cost of exterior paints reflects greater resin content and costlier non-fading pigments. You’re not likely to find these paints priced low.
Of course, a high price doesn’t always reflect high-quality ingredients. That leaves the homeowner with a dilemma. How do you know what you’re getting?
Paint formulas, when they’re listed on the can, are too complicated to decipher. You really do have to rely on the integrity of regional and national brand names and the assurance of your local paint dealer.
What is paint chalking?
Chalking is the appearance of a light, powdery dust on the surface of the paint, which dulls the color. The chalk is mostly pigment left on the surface as the resin binders slowly break down and erode.
The chalk itself won’t cause trouble unless rain washes a colored pigment down onto a different-colored siding or over bricks, staining them. Water-based acrylics generally don’t chalk, which is one of their advantages for exterior applications.
Are there good applications for inexpensive exterior paints?
When you paint a ham or other outbuilding, you don’t need a paint that looks great, just one that protects the wood.
For these paints, formulators use inexpensive oils like linseed oil for binders, and inexpensive pigments like iron oxide. Oils dry to a film slowly but remain flexible and durable.
The low cost of iron oxide, your basic rust, helps explain why most barns are red.
How can I paint wooden shingles and resawn wooden shingles
Rough surfaces exposed to weathering, especially wood shingle or shake siding, present another problem for paint films. (Shakes are thicker than shingles.)
These materials have more exposed surface, which gives rain more places to penetrate the wood. Wet wood will eventually cause even the best-quality paint to blister and peel.
Even worse, peeling paint on any rough-textured wood presents a difficult scraping job when you prepare it for repainting.
Water-based acrylic siding paints work better than oil-based paints on rough textures, because they “breathe,” allowing trapped water vapor to pass through them more easily.
They’re also more flexible when humidity changes cause the wood to expand and contract. Make sure you prime bare wood first (as always) with an exterior primer to ensure good adhesion.
If you don’t want to risk peeling paint on a rough exterior surface, you might want to consider exterior stain as an alternative (Fig. B).
An exterior stain is a very thin paint, though it can be loaded with enough pigments to completely coat the wood (called a “solid” or “opaque” stain). It doesn’t contain enough binder to form a film, so it won‘t peel. And you won’t have to do as much prep work before recoating.
Stain doesn’t last quite as long as paint on rough surfaces, and it’s not available in paint’s bright colors.
What is the best paint for porches and floors?
When trying to come up with a good paint for an exterior floor, you catch a paint formulator between a rock and a hard place.
Good exterior coatings must be flexible enough to withstand moisture and temperature changes, but on a floor, they have to be hard enough to withstand the scuffing and abrasion of foot traffic.
So porch and floor paints are resin-rich gloss paints (usually called enamels) sometimes made with harder resins. Water-based acrylic resins work well. Oil-base alkyd floor paints are often modified with even tougher resins like phenolics.
But any paint can have trouble adhering to a porch floor because water can sit on its surface for a long time, eventually soak through, and break the film’s bond to the wood.
For best results, be sure to apply an exterior primer first, or thin the first coat according to directions on the can.
Unfortunately, the gloss finish resulting from higher resin content can make wet painted floors slippery, so you might want to add fine sand to the paint to improve traction.
The air-quality laws that limit the solvent content of finishes posed an immediate problem for floor finishers who used polyurethane for a clear finish on interior hardwood floors.
Paint chemists quickly marketed a new line of water-based clear finishes that still contain such excellent abrasion-resistant resins as urethane. The best of these cost up to $80 per gallon, but they perform as well as or better than oil-based floor finishes.
We can expect this technology to expand into pigmented finishes, and you’ll begin to see water-based paints containing some of the excellent resins once exclusively used in oil-based paints.
What is the best paint for concrete?
Concrete presents paint films with two challenges: moisture and alkali.
Concrete is porous, so as everyone with a basement knows, moisture moves through it easily.
Moisture in concrete will break a paint film’s bond to the surface and cause it to peel. No paint system will adhere to damp concrete for long. If you want to paint it, you have to make sure the concrete remains dry
Water-based acrylic paints have an advantage here because they breathe and won’t trap moisture as readily as oil-based paints.
Alkali is a corrosive substance present in all concrete, especially when newly poured. You can neutralize it, however, by washing the surfaces with a diluted solution of muriatic acid.
Wear goggles and rubber gloves when working with acids, even weak solutions like this.
In addition, for walls use a special water-based concrete primer, which contains PVA (polyvinyl acetate). PVA is actually the standard resin used in inexpensive latex paint. Most paint stores sell this primer.
PVA is too soft for concrete floors, so skip the primer on floors and apply two coats of water-based acrylic or an acrylic modified with epoxy resins. Epoxy resins deteriorate under intense sunlight but perform well indoors.
Interior paints don’t face the same rigors as outdoor paints, so paint formulators can make them from lower-cost resins and pigments. This is why you’ll usually find interior paints priced lower than exterior paints.
Furthermore, most people prefer wall paints with a “flat” luster, that is, with no gloss. Flat finishes hide imperfections in the wall surface, while gloss paints highlight every irregularity. Flat paints contain less resin than gloss paints (Figs. B and D), making them even more economical.
The amount of pigment in paint usually determines the smoothness of the film surface.
Unfortunately, the rough surface that makes a paint film look “flat” also catches dirt, fingerprints, and streaks from every toy your kid bounces off it. And while almost all wall paints are washable, low-cost films quickly wash away, leaving you with a repainting job.
Truly washable paints must have a smoother surface, that is, contain more resin like a semi-gloss paint. Paint formulators found that they could make a smooth, washable surface without increasing the resins by adding different types of pigments (as fillers). These paints are called “eggshell” paints.
When you paint metal for outdoor use, you generally run afoul of two problems: making the paint stick and stopping corrosion. Unlike wood, which is porous, metal surfaces are smooth, even polished, so paint can’t get a good grip on the surface.
Paints stick better to metal if you roughen the metal surfaces and use special primers formulated to adhere to bare metal.
The second problem is stopping corrosion, especially from the water. Water causes metal surfaces to react, corrode and flake, breaking the paint bond and causing it to peel.
Special primers also help stop corrosion. Oil-based and other solvent-based paints perform well on metal because they form a watertight film and adhere well.
There are a few exceptions, however. Don’t use an oil-based primer on galvanized steel. Paint your galvanized gutters and downspouts with a water-based primer designed for galvanized metal and then add an acrylic topcoat.
Proper prep work is critical here. Wipe all oil and grease from the surface, especially new surfaces, with a strong solvent like xylol (available from painting stores). Solvents like paint thinner and turpentine won‘t work, because they leave an oily residue.
Do your prep work outside where you won’t have to breathe concentrated fumes, and wear rubber gloves.
Paint won’t stick well to aluminum unless it’s chemically treated first (anodized). Consult a paint specialist for help on this one. On the other hand, aluminum siding already has a good, baked-on prime coat, so simply paint over it with an acrylic exterior paint.
Some types of wood, when used outdoors, won’t hold paint well. Most hardwoods, like oak, maple and birch, fall into this category, but so does Southern yellow pine.
The darker grain lines in this pine species are dense and resinous. Paint doesn’t adhere well to the grain lines outdoors where the wood faces weathering. So builders often use this wood for porch ceilings and underneath overhangs, places where it won’t get wet.
Douglas fir plywood, the most common type of plywood, doesn’t hold paint well outdoors either, partly for the same reason.
In addition, the surface veneer contains many tiny surface cracks that quickly fracture the paint film. An exterior stain is often a better choice than paint. It won’t last as long, but you won’t have to scrape off a peeling paint film when you recoat the wood.
Redwood can also cause trouble. Its red heartwood contains natural tannins (acids) that can leach through paint and leave black stains on exterior siding, decks or fences. So when you paint redwood, use a primer specially formulated to block these stains.
Finally, hardboard exterior siding, often used over the last 25 years, will quickly deteriorate if water penetrates the paint film. Hardboard is made from compressed wood fiber and resins. If it’s not protected, the exposed fibers quickly soak up water, swell and further fracture the paint film.
The best solution is vigilance. Watch for swelling or signs of paint failure, especially at the bottom edges of boards and at joints. Then sand, prime with an oil-based primer and recoat them immediately with a high-quality acrylic topcoat.