How to Paint Fiberglass
The article is a how-to broken down by surface type (polyester or epoxy), paint type (one or two part), tools, and techniques.
Why Paint Fiberglass?
Fiberglass (Fiber Reinforced Plastic) is often susceptible to water incursion and/or sun damage, depending on the resin used. Epoxy weakens, with time, in sunlight. Polyesters are subject to water infiltration and vinylesters, though stronger than polyesters, are not as strong as epoxy.
Water incursion will cause blistering or bubbling, though this typically takes years to develop. However, once blistering appears it's only a matter of time before the composite (fiberglass and resin) delaminates, and you have major problems on your hands. This is an especially important concern in boating.
Sunlight, usually with epoxy resins, will cause the composite to crumble. As you might imagine, this is pretty bad.
Paints are used to add an additional barrier against both water and ultraviolet light (sunlight). In the “finish business,” these are called industrial coatings. A good industrial coating provides a protective layer against these incursions, and they look good too. For this document, I will simply stick with the “paint” designation, but if you hear “industrial coating,” now you’ll know what is being referred to.
In this paper, I will recommend two types of paint (not brands) and cover the pros and cons of using each type. No single coating provides perfect protection. Along with this information, I'll also recommend two painting techniques. Any paint type covered here can be applied with either technique. So the paint type and method of application are completely independent of one another.
Two Types of Coatings Suitable for Fiberglass: Polyester- Versus Epoxy-Based
FRP stands for Fiber-Reinforced Plastic. There are two major categories of paint useful on FRP. They are both polymer (or plastics) based. One uses exposure to air to cure and the other a catalyst, temperature, and air. To be clear, air-cured paints are typically polyesters. They only require the “one-part” that comes in a single container. Catalyst-cured paints are referred to as epoxies; they come in two parts that are either equal in volume or slightly disproportionate quantities. Epoxies require a catalyst and, like “poly,” the right temperature range.
Because these paint types are basically plastics in a volatile liquid, they adhere well to the three resins used in creating fiber-reinforced plastic objects.
Any resin-based paint is suitable for application on FRP, but I recommend using epoxy-based paints with epoxy-based FRP resins and polyester-based paints on polyester-based FRP. I make this recommendation because like materials bond better. To put this another way, an epoxy paint bonds best on an epoxy-fiber reinforced plastic. Alternatively, polyester paints adhere best to polyester-based plastics. Similar plastics bond better to each other.
I will make an exception to this recommendation. Should you be painting a boat or other FRP object that is going to be subjected to temperature extremes and/or water immersion, I highly recommend epoxy-based paints regardless of the resin used in the FRP. Epoxy is simply a much more robust paint, particularly when that coating is subjected to salt or fresh water and near constant bumps and dings.
Epoxy, as long as it is protected from sunlight, is both strong and flexible. The pigments in epoxy-based paint will provide the ultraviolet barrier needed to protect the fiberglass beneath. However, if you store an epoxy-painted object outdoors, I recommend covering it with a tarp or fabric to give it some additional protection from the UV rays in sunlight.
What Is in the Paint
Paint typically consists of a vehicle (liquid), a binder, and pigment. In paints used for durable plastic coatings, these are volatile organic compounds (liquid carrier), polymers (binder), and pigment (color).
I do not recommend powder-coat due to the high temperatures required to bond this coating. Since powder-coats require temperatures of 300-350°F, fiber-reinforced plastics will almost certainly melt at those temperatures or, worse, catch fire.
Volatile Organic Compounds
A component of most of these paints is Volatile Organic Compounds (VOCs).
VOCs are liquids designed to evaporate at a set rate, usually quickly. The idea is for the liquid to remain fluid long enough for the paint to self-level, yet evaporate quickly enough to create a durable coating in a short amount of time. These compounds are not good to breathe, and appropriate protective equipment should be used while applying them.
Both one-part polyurethane and two-part epoxy paints use VOCs.
Another component in polymer paints is the binder.
Binders are the chemical compounds that cause the pigment particles to bind to each other. The binder is typically a plastic including resins such as acrylics, polyurethanes, polyesters, melamine resins, epoxy, or oil. Note that only the bold-faced materials mentioned work well on fiberglass.
Pigments comprise the smallest percentage of paint. Though most modern pigments are chemically derived, manufacturers tend to keep the actual sources and formulations a trade secret. That means that knowing exactly what is in your paint may be impossible to determine. For that reason, you should always consult the paint dealer if you have any questions about what surfaces to use the paint on.
Preparations for painting are almost everything; these include taking into account the temperature, humidity level, placement relative to direct sunlight, and even wind conditions.
Relative humidity should be below sixty (60%) percent and temperature should be between 65° and 90° degrees Fahrenheit or 19° to 35° Celsius. Note that the lower the temperature (within the range), the longer the drying or curing time.
If the ambient temperature is too low, the paint won't harden or cure. If the temperature is too high, the paint will set almost instantly, leaving the layer just below in a liquid state. The result is a very poor paint job.
Sanding and Cleaning
Surface prep is very important. In fact, prep should comprise about 80% to 90% of the job. Paint will not stick to dirty, oily, or greasy surfaces.
Paint will not adhere well to glossy surfaces either, so sanding is a vital step.
Methods of Applying Paint
There are two main methods of applying paint to various surfaces: spray painting and brush painting. These methods are geared toward the home hobbyist or the adventurous hobbyist—the best one for you depends on the equipment available to you and/or your experience.
The paint is applied with a compressed-air/or pumped-pressure spray gun. This requires a compressor and a good-quality spray gun, as well as paint filters, paint stirrers, eye and hand protection, masks (you don't want to breathe the stuff), solvents, containers, and plenty of clean rags.
Roll and Tip Method
The paint is applied with a roller and then followed up almost immediately with the tip of a fine bristle or foam brush. Tools requirements are a solvent resistant roller, solvent resistant roller covers, brushes, containers, paint filters, eye and hand protection, stirrers, solvents, and plenty of rags.