Tips for obtaining good results when working with these paints
By Joe Escobar
In the area of aircraft painting, high solids paints are being used more and more frequently. Some facilities are already using these paints, and others are considering switching over to them. The choice to use high solids paints can be based on regulatory requirements, manufacturer specifications, or environmental concerns. In this article, we will take a look at some of the issues involved when working with high solids paint products.
What is a high solids paint?
High solids paint is a term used to identify a paint that is formulated to have a higher concentration of resin and a smaller concentration of volatile organic compounds (VOC). Most aircraft paint manufacturers offer some type of high solids paint product. These paints have been developed to meet stricter emissions regulations passed since the mid-’90s by the EPA and local and state regulatory agencies. In particular, the EPA passed the national emission standards for hazardous air pollutants (NESHAP) for aerospace manufacturing and rework facilities that limits the amount of VOCs and hazardous air pollutants (HAP) that are allowed to be emitted to the atmosphere. The compliance date of NESHAP was Sept. 1, 1998.
The strict regulations of NESHAP do not specify that high solids paints be used in all applications. Instead, it sets limits on emissions that are permissible. Although the applicability of the standard’s strict limits in emissions is applicable to everyone, that may not require you to use high solids paint if you can show that you will stay under the threshold for emissions limits. Many facilities have shown that they comply with the standard while still using traditional paints. However, many facilities are using high solids paint to reduce their VOC and HAP emissions. Factors that affect the amount of VOCs and HAPs emitted are numerous including the paint process and amount of aircraft being painted.
So, if high solids paints are more environmentally friendly, why hasn’t everyone switched over? Well, several factors play into the decision. High-solids paints tend to be more expensive than traditional paints. However, this factor may not affect the total cost of paint since less product is needed to apply the same paint coating thickness. This is due to the fact that high solids paints are comprised of a greater percent of resin than traditional paints, leading to less volume needed to adequately cover the same area.
Another factor that may affect the decision to use high solids paint is the difficulty working with the paint. If the painter is not familiar with the proper process when using high solids paint, the end result may be less than desirable. Many painters express frustration when first working with high solids paint. Incorrect application of the paint can seriously affect the quality and appearance of the paint job, and the decision may be hastily made not to continue working with the high solids paint. If painting with traditional paint can be considered an art form, then working with high solids paint can often seem like trying to duplicate a classical painting using a paint-by-number kit. Here are a few tips for ensuring your work of art looks more like a classical painting and less like a paint-by-numbers piece.
Just like with traditional paint, proper surface preparation is essential to getting a good end product. We will briefly cover some basic surface preparation tips to help obtain that quality paint job.
There are different methods of paint removal that are used. We will not discuss the different methods of paint removal here, but will instead assume the aircraft has already been stripped according to approved methods and that we are starting with bare metal.
To obtain best results, all aluminum skin surfaces should be lightly scuffed prior to surface treatment. The surface treatment consists of applying an approved acid etch followed by a conversion coating. This is then followed by applying an anti-corrosive primer to the surface. Proper performance of these steps greatly improves adhesion of the paint as well as corrosion resistance, and is the foundation for a good looking paint job.
The surface of the aircraft should be scuffed using abrasive pads, using care not to exceed minimum thickness of the metal or damage fasteners. The aircraft should then be rinsed down. Acid etch should then be applied to the aircraft surface. The acid etch performs several functions. It helps cleanse the surface of contaminants. It also removes any oxidation on the aluminum. Another important function of the etch is that it creates a microscopic profile in the metal to aid in adhesion. Follow the recommended procedures for the etch. Also, follow the aircraft manufacturer’s instructions for protecting those components that should not be exposed to the etch or the subsequent chromic acid process such as composites, plastic, and other materials that can be damaged by the acids.
Usually, the acid etch is applied to the aluminum skin by using a scrub brush or mild abrasive pad. Proper application of the acid etch is visually identifiable by a bright aluminum surface. After a very brief dwell time, the etch is rinsed off using fresh water. When rinsing the etch, the water should sheet completely over the skin in a thin layer. If there are any areas that bead up, as with a freshly waxed car, this indicates that area was not properly treated and the etch process should be repeated there. Be sure not to let the etch dry on the surface at any point during the etching process.
The etch process is followed by applying a chromic acid conversion coating. This is generally known by the trade name Alodine. The conversion coating is applied immediately after the etch rinse. After a few minutes of dwell time, the surface of the skin will turn a light gold color. When this happens, the surface should again be rinsed with fresh water ensuring all chromic acid is removed. If the chromic acid is allowed to dwell too long, or if it is not rinsed completely off, then the skin will turn a dark brown color, and this could adversely affect proper adhesion of the paint. Special attention should be paid to rivet heads and lap seams during the rinsing process to ensure no residual chromic acid is allowed to remain in those areas.
The primer should be applied as soon as possible after the chromic acid has been rinsed off and the surface has dried. Be sure to follow the product guidelines, but most suggest that the primer not be applied in an excessively heavy film. Doing so compromises adhesion of the primer. One thin pass is usually adequate in primer application. This will usually be evident as a translucent coat on the surface. A light scuff is usually recommended on the dried primer prior to topcoat application. Just prior to topcoat application, the surface can be wiped with a clean rag dampened with an approved solvent to remove any dust and contamination. In addition, the surface can be wiped with an approved tack rag for final dust removal.
There are many factors that can affect the quality of the topcoat application. Assuming a proper preparation has been accomplished, here are a few tips on applying the topcoat.
Use clean equipment. The spray equipment should always be thoroughly cleaned. High solids paints have a thicker viscosity than traditional paints, and any contamination in the paint gun can adversely affect the quality of the spray. If you run a few ounces of thinner through the gun prior to use, you can ensure all of the passages are clean. This is also a good way to ensure the spray fan is even and that the spray gun is functioning properly. Be sure to use the proper tip, needle, and air cap combinations for your paint gun as recommended by the manufacturer in order to obtain proper atomization of the paint.
Use proper air pressure. For best results, paint guns require proper air pressure to obtain proper atomization of the paint. High pressure is usually necessary for applying high solids paints because of the increased amount of resin present, and resulting in higher viscosity of the paint solution. The recommended pressure is usually 55 to 60 psi. One thing that some painters don’t realize is that significant pressure reductions occur over long air hose runs. So even though your pressure gauge at the air source is within limits, if you are using a long air hose, the actual pressure at the paint gun may be significantly lower. Always check the output pressure at the hose connection to the gun, and adjust pressure as needed to obtain proper operating pressure at that point.
Use a clean air source. Ensure your air supply is clean, dry, and free of contaminants. There are numerous commercial dryers and filters available to help maintain a high quality of compressed air. If manual oil/water filters are used, ensure they are drained regularly to avoid accidental introduction of water or oil into the system.
Mix paint properly. The paint should be mixed according to manufacturer’s recommendations. Always use a good paint filter when filling the paint pot. You want to strain the mixture into the paint pot using an extra-fine paper cone filter.
Apply coating in proper thickness. Applying the paint in proper thickness is essential to a successful paint finish using high solids paint. Usually, two passes are adequate for proper coverage, although some lighter colors may require three passes. The first coat should be applied so there is adequate wetness on the surface. A shiny mirror finish will be evident on the first pass within a few minutes with proper application. The sub-surface may not be completely hidden, especially with lighter colors. If too much paint is applied on the first coat, then a substantial amount of drying time will be necessary before the final wet coat is applied. If it is applied too lightly, then dry areas will develop with a granule-like texture that may not flow out when the final wet coat is applied. The second, or wet, coat should be applied after the first coat has dried to the touch. The time it takes to dry to this point will vary based on temperature and humidity. If the initial coat has not dried adequately before the wet coat is applied, this could lead to orange peel — one of the leading frustrations painters encounter when working with high solids paint. The wet coat should be applied in a thin, uniform layer that flows smoothly a few minutes after application into a high gloss.
Getting good quality results when using high solids paint can seem like mission impossible when first exposed to it. There are many factors that can affect the quality of the finished product. This article has briefly touched on some of those factors. But in order to ensure you get the best quality paint job, you should consider receiving training from the paint companies that offer high solids paint products. They are extremely familiar with the product, and can train you on all the factors to keep in mind when working with these paints. The paint manufacturers deal one-on-one with many maintenance facilities, and will often be able to figure out any problems you may have in your process that can be leading to less-than-desired results. The knowledge gained by communicating with and receiving training from the paint manufacturer can go a long way in ensuring your high solids paint process is as trouble-free as possible. This can ensure your work of art is a true masterpiece.