Turn out your best work with a nuts and bolts understanding of these paints
Solvent-borne paint has four basic components: pigments, rosins or binders, solvents and additives. Each of these components contribute significantly to the performance of the coating, either as it is being applied or during its lifespan (holdout).
Pigments
Though most painters think of the pigment as the "color" (and often it is), pigment also contributes to the hiding ability of a coating, its filling properties (the filler in primer filler is technically a pigment) and its sandability, adhesion, corrosion resistance and durability. The pigments that create color are finely ground naturally occurring minerals or manufactured products that add to the color and its opacity (how well it hides or covers). Pigments also add to the coating's glamour with metallic, pearls (mica) and color shifting materials. Other "pigments," called extenders, provide properties such as filling and sandability as in primers, adhesion and corrosion protection. Most extenders are translucent and therefore do little for opacity. Pigments make up the bulk of the solids in the paint. When a coating is referred to as a high-solids paint, it is high in pigments, the substance that remains after the paint cures. It is these solids, with the help of rosins, that not only look good after they are cured but provide protection and long-lasting resistance to the elements – a vital part of a coating's job.Rosins
Rosins, sometimes called binders, do just that. They bind the pigments to the substrate below it. These are the film-forming components of a coating that help provide gloss, durability, flow (rheology) and adhesion.
In the past, rosins were produced from oils (linseed oil, from flax, for example), and then from tree saps, such as varnish. In more modern times, rosins were produced from synthetic hydrocarbons (petroleum). Now we have many rosins to choose from. The type of rosin determines the durability and other properties of the coating. Rosins such as acrylic urethanes, epoxy and polyester are each suited to specific situations.These different rosins have their advantages and disadvantages. For example, epoxy has excellent adhesion qualities and when teamed with a corrosion resistant pigment provides an outstanding corrosion-resistant base. Its disadvantage is that it takes time to apply. Therefore, many collision repair businesses choose a vinyl wash primer (acid etch) for their direct-to-metal corrosion protection.
The way that paint systems deliver their rosins also differs between manufacturers. Some are called "balanced" systems. That is, the toners on the mixing rack have the rosins in them and can be taken off the shelf, reduced and applied as a basecoat. Others do not have rosins in the toners and must have one added before they can be applied as a basecoat. Some manufacturers provide a mixture of balanced toners and non-balanced ones. Knowing your system and following the prescribed recommendations is very important.Solvents
This category of paint components is highly misunderstood. Though painters use it each time they reduce paint and should be making choices as to which one they should be using, depending on the spray conditions, they often try to make choices without knowing how solvents work.
Solvents are called the coating's "vehicle" by paint chemists. Solvents both dissolve the rosins and suspend the pigments, making it possible to move the paint from the mixing table, through the gun and onto the vehicle where the solvents evaporate.
Paint thinners or reducers are chemical compounds that evaporate at different rates. These rates control liquidity and help regulate drying (evaporation) and curing. Paint solvents evaporate at three differing stages. The first stage involves evaporation from the time the paint leaves the gun until it reaches the vehicle. This is called "in flight loss of solvent." Because the air propels the paint, certain types of solvents evaporate very quickly. Chemists call the speed of evaporation of a chemical its volatility.
The second stage is called the settling or flow-out stage. During this time paint remains liquid (wet), and the coating's atomized droplets reconnect and flow over the part. Painters must wait for a prescribed "flash" time to transpire, or defects such as runs or solvent popping may occur.
During the third or "tail" stage, the solvents evaporate, with some taking as long as 90 days. Paint, like concrete, is hard and quite durable and will not fully evaporate for some time. During this stage, paints cure either by oxidation or chemical cross-linking.
Paint companies provide different reducers for mixing paint under different conditions. Often they are categorized by temperature. They may be called fast, medium or slow reducers. Manufacturers often provide guidelines for mixing under specific temperatures but seldom refer to the humidity as a consideration. If the air becomes saturated with moisture (for example, 90 percent humidity), solvents may evaporate more slowly due to the saturated air. Under these conditions, a faster-evaporating solvent may be better for the conditions.
Painters don't always understand the impact of humidity on flash time. Sometimes a manufacturer recommends a flash time five to 15 minutes before subsequent coats. This raises the question: When should we only wait five minutes and when should we wait 15? Often painters wait only the minimum time, hoping to speed up cycle time. But a wiser consideration would be that on a hot dry day, the five minute wait is appropriate, and on a more humid day, 15 minutes could well produce better results.
Additives
Additives, the last components of paint, add enhancements such as anti-gelling (longer shelf life), UV resistance, anti-foaming and other special performance qualities. One component that is added to paint before spraying is the catalyst or hardener. With paints that chemically cure, the catalyst helps with the chemical reaction called crosslinking. Though some paints crosslink when exposed to oxygen (enamels), others, such as acrylic urethanes, require the addition of a hardener to fully cure. These paints, without a hardener, will dry but not cure. Hardeners have matured over the years. A selection of speeds at which they cure paint is now available in most paint systems.
Curing
There are four main ways that paints cure: evaporation, oxidation, chemical reaction and radiation.
Lacquer paints evaporate and are the only paint systems that truly dry. Because of this quality, they may be fast to apply but are not very durable. Due to their low durability, as well as the danger they pose to the environment (large amounts of solvent must be used), they are not used much and are not even available in some areas.
Though they contain solvents that evaporate, enamels cure by crosslinking (paint chemicals connect to form a strong film) when exposed to air; oxidation requires both oxygen and time to cure. The curing time is decreased only slightly with the introduction of heat. Enamels are more durable than lacquers but not as durable as urethanes.
Chemical reaction curing paints cure by chemical crosslinking. The added hardeners provide the chemicals for this crosslinking. They bring the parts of the liquid paint together, providing a strong durable finish. This chemical reaction is increased when heat is introduced, accounting for the increased popularity of baking spray booths in the 1980s. Though hardeners are normally associated with clearcoat finishes, basecoat is often catalyzed or hardened to meet manufacturers' refinish recommendations.
A positive effect
So, how does knowing paint "chemistry" help the everyday painter? Actually a painter has a great deal of control over paint and how it will act. The three areas that affect paint application performance are air movement, temperature and humidity. The standard for most paint companies is 80 CFM airflow, 70 degrees F and 50 percent humidity. Each paint company lists its standards, and painters should know them.
Painters have some control over the air movement in a spray booth. If they maintain the filters and balance the booth to slightly positive, air movement should remain constant, though downdraft paint booths generally have higher airflow than crossdraft or semi-downdraft booths. Some paint manufacturers recommend that a painter use a reducer the next level slower in a paint booth with high airflow.
As for temperature, in a baking booth one has a great deal of control. The painter can set the temperature that affords the best results with the reducer being used. Some painters like a booth set at 80 degrees F to paint, which for many (myself included) would be too high. If 80 degrees F is the temperature a painter prefers, then he or she must adjust the solvent to a slower one. The flash time must be about seven to 10 minutes long. This time allows the second gassing out of the settling solvents. If this settling time is too short, the coating will not flow, which affects both metallic lay and texture. Remember, the hotter the temperature, the slower the reducer. This practice is aimed at keeping the flash (settling) time constant.
Humidity is the area over which we have the least direct control. Yes, as we use the heat source it dehydrates the incoming air. But in the summer, when the air temperature and the relative humidity are both in the 90s, setting the booth to 120 degrees F to drive off the humidity is impractical. If the humidity is at 90 percent, that means that the air surrounding your project is 90 percent saturated and the solvent that you want to gas out (evaporate) will do so at a much slower rate than if it were 50 percent, the standard for your listed flash time. Even adjusting the solvent to a faster evaporating one may not be sufficient. The painter is left with the need to extend the flash time.
Flash times are often listed as a range, such as "5 to 10 minutes." That is a large variable. When does a painter know to let the paint gas out for five minutes, and when should it be allowed 10? Though reducer choice does affect flash time, humidity does as well and to a greater degree. As the humidity rises, flash time should be extended. That means in the wintertime, as humidity goes down and with paint booths that have temperature control, the flash time will also go down. As humidity rises, so do flash times.
Painting is often done by instinct, and a person who paints must have good instincts to succeed with today's paint technology, expense and time constraints. The more knowledge painters have about the products they use, the better and faster they will perform. A constant trend in collision repair is to do more in less time, with better quality. Successful painters in this climate know, don't guess, how to best use paint products.
As paints become more sophisticated, painters too need to be even more sophisticated when using them. You've heard of early pilots flying their biplanes "by the seat of their pants?" That doesn't work here. I want my painters — using today's materials — to be highly trained and knowledgeable. Painters need to know as much as they can to make today's basecoats perform as they should — consistently.