Paint selection guide

Acrylic or polyurethane? Enamels or lacquers? WhatÕs the best paint for your restoration project? To help you answer that question, hereÕs a quick paint guide.

Acrylic enamels: Enamels produce a highly durable and weather-resistant finish. They dry more quickly than straight enamels, resist scratches, and have a high gloss finish. Such paint is often sprayed over an epoxy primer.ÊA light, mist coat is first sprayed on and allowed to dry for a few minutes until it is tacky to the touch. The mist coat is then followed by a full coat of enamel. One full coat may be sufficient or another may be sprayed if desired. Acrylic enamels dry more slowly than lacquers, and often require color sanding and buffing to bring out luster.

Acrylic lacquers: Such paints produce a hard, quick drying, durable finish but do not resist chemical or fuel spills. But acrylic lacquers have a low solid content, which can make the paint more difficult to apply. With lacquer paints, apply a very light tack coat followed by four to five additional coats of paint, and allow about 30 minutes drying time between coats. The disadvantage with lacquers is that not providing for proper drying time between coats can produce cracks in paint finish. Lacquers must be buffed to bring out luster.

Polyurethane enamels: Polys are probably the most popular choice for show-quality topcoating. They are very durable, produce a high gloss finish, and are resistant to most chemicals and fuels. Poly paints have a high solids content. And these paints cure very slowly, which means they continue to flow out for a long period of time. This later characteristic results in a flat surface that provides a high gloss look. Polyurethane enamels are mixed with a catalyst prior to use and then reduced (thinned) to proper viscosity for spraying. During application a very light tack coat is first applied followed by one or two coats. You will want to avoid overapplying poly paints because thick top coats crack over time. Finally, wearing a mask equipped with a filter specifically designed for polyurethane painting is a must when spraying the material since breathing the mist from the paint can cause severe sickness or even death.

Mixing the catalyst in polyurethane paints should be done in accordance with the directions from the manufacturer. Generally, the catalyst should react with the base paint for at least 20 minutes prior to spraying. Once the paint has been mixed you will have approximately five hours before chemical crosslinking begins and the mixture begins to thicken. With this in mind, only mix the amount of paint you will need for the job. If too much paint is mixed, you can place it in a freezer (not with food) overnight, remove it, and allow it to reach room temperature before spraying. The cold temperature will delay the crosslinking process. In addition to reducers, other chemicals you may encounter are retarders and accelerators. A paint retarder, very simply, is a solvent that slows the drying time of the paint. Retarders are added in proportion to the directions on the paint can. Retarders are generally used in high-temperature or high-humidity conditions. Accelerators have the opposite effect - they speed up the drying time. Accelerators may be needed when spraying in cool temperatures.

No matter which paint you choose, certain fundamentals apply:

• All coatings must be thoroughly mixed. This includes shaking the paint on a commercial mixer within a week of application as well as stirring the coating just priorÊto use.

• Wipe all surfaces to be painted with a cleaning solvent and lint-free tack cloth to remove dust.

• The paint should be properly thinned by following the manufacturerÕs directions.ÊA viscosity cup can be used for the thinning procedure. A viscosity cup is a small cup with a hole in the bottom. The cup is filled with paint and the viscosity is determined by the amount of time, in seconds, required for the paint to flow through the hole until it begins to drip.

• The paint should be strained through a mesh strainer prior to placing it in the spray cup. When using a polyurethane paint, strain the paint prior to mixing the catalyst.

The basic ingredients of paint

Paint consists of pigment, binder, thinner and other additives. But what do these additives offer in paint? Well, we all understand that paint pigment provides color. But pigments also provide paint coverage as well. In addition, it protects the binder from weathering such as from ultra violet rays. Also some types of pigment control the gloss of the finish coat. There are two basic categories of pigment. They include hiding pigments of white (titanium dioxide), red, yellow, black, brown, blue, organic, and oxide colors, and non-hiding pigments of talc, clay, calcium carbonate, and silica.

Binders, as the name implies, hold pigment together. In this regard, the binder acts as a vehicle that carries pigment when applied and provides adhesion to the surface to which it is applied. The type of binder used in paint determines the durability of the paint product. Common binders used in paint include linseed oil, alkyd, polyurethane, acrylic, and poly-vinyl acetate (PVA). Each of these binders has unique characteristics and can be used alone or with other binders to complement each other.

• Linseed oil is used in exterior applications only and creates a flexible oil film that aids in preserving weathered wood.

• Alkyd is used in both interior and exterior paint. Alkyd resin is very durable and forms a hard film commonly used for heavy-duty applications.

• Polyurethane has two common uses on automotive coatings and for products that come in contact with concrete (household and industrial use).

• Acrylic resin is a waterborne latex. Acrylic is used almost exclusively for exterior paint coatings resulting in paint that is very flexible and weather-resistant for exterior use. There are oil base acrylics used in the automotive industry, but those should not be confused with acrylic house paints.

• PVA is also a waterborne resin used mainly in interior paints. PVA can be used in exterior paints either by itself, or in combination with acrylic. Exterior moisture resistance and weathering of PVA will not be as good as acrylic.

Thinners are great for cleanup but serve a vital role in paint since they combine the pigment and binder to control consistency. And thinners plays a roll in the paint film drying. The most common types of thinners used in paint manufacturing include mineral spirits (used in most oil formulations), VM&P naphtha (used in some spraying enamels), water (used in latex formulations) and lacquer thinner (used in lacquer varnishes). Finally, additives come into to play to regulate application properties such as brushing, rolling, spraying, etc. Some additives help some product formulations dry while others aid in pigment dispersion in manufacturing or when adding liquid colorant. And then additives determine various paintsÕ film formation properties.

Different combinations of these paint ingredients can produce a dry paint film that offers certain characteristics. For example, a flat finish paint is usually created from a mixture of 70% pigment and 30% binder. This higher load of pigment allows the pigment to be present at the exposed surface of the paint. This high percentage of pigment on the dry paint film surface causes the light falling on it to scatter, so as to be substantially free from gloss or sheen. Gloss paints, on the other hand, usually have a 20% pigment to 80% binder ratio. Gloss paint provides a more durable finish because they contain less filler or extender pigments, which are usually soft. Satin and semi-gloss will normally fall somewhere in the middle of gloss and flat finish paint in pigment-to-binder ratio and durability. Combination of paint ingredients also affects how paint drys. For example, lacquer-based paints dry via the evaporation of the paint solvent. Linseed oil paints dry via oxidation and polymerization. Latex paints dry with evaporation of water and fusion. And oil- and expoxy-based paints dry via chemical catalyzation.

One common misconception is that lacquer thinner or other strong solvents can be used to speed up paint dry time. In most cases, this is not so. If thinning is needed, use the thinner recommended on the label of the paint. If strong solvent is added, uneven gloss, wrinkling, and dissolving of previous coatings are only a few of many problems that can be caused. All thinner must evaporate from a coating before a film can begin to cure. Solvent is normally added to change consistency for different application methods.