by Ron Joseph
Zinc-Rich Primer vs. Conventional Primers
Q. We have been asked to refurbish some trailers, baggage carts, and other miscellaneous ground support equipment used at airports, using an inorganic zinc-rich primer. We have never used such as primer and would like your comments please.
It is just as well you asked, because there are several things you need to consider before using a zinc-rich primer. Here are some of the most important, but please bear in mind that the explanations given below are not exhaustive. I'm sure there are more pro's and con's that can be added.
Zinc-rich primers provide outstanding corrosion resistance, especially in aggressive corrosive environments
Zinc-rich primers are intended for structural steel members and not for thin-gauge metal sheets, such as you might come across on your equipment.
You cannot apply a zinc-rich primer to previously painted surfaces. The primer must be in intimate contact with the clean metal, otherwise it will not provide cathodic protection to the metal.
You should not apply the primer to surfaces that contain any greases or oils.
All surfaces must be abrasive blasted to a white or near-white metal finish, and paint vendors usually recommend a blast profile of approximately 1.5-2.0 mils. If sheet metal were to be prepared in this manner, it would probably deform. Hence, the previous statement that zinc-rich metals are intended for thicker structural steel members.
After abrasive blasting the metal, you should apply the zinc-rich primer within a few hours - well before the metal can start to show signs of flash rusting.
Abrasive blasting is often not recommended or convenient when performed on existing equipment that contains motors, bearings and moving parts, because the abrasive can often find its way between the moving parts.
Inorganic zinc-rich primers come in two parts; a small quantity of a clear silicate resin in one can, and a large volume of zinc powder in the other. The difficulty faced by the painter is to intimately mix the zinc with the silicate resin. The goal is to have each individual zinc dust particle coated with the resin, but this is no mean feat if one considers how much zinc is added to the small volume of resin. Therefore, suitable mixing equipment is required and the painter must be properly trained and preferably have previous experience in this application.
Application of the zinc-rich primer to the surface is also critical as it tends to go on dry. An experienced painter knows how to apply the primer to achieve a wet film that will later provide excellent corrosion protection. If the coating is applied too dry, the powdery top layer is a poor base for any subsequent topcoat.
HVLP spray guns are not suitable for the application of these primers; instead airless or air-assisted airless guns are preferred because of the higher fluid pressure generated by the hydraulic pumps.
The orifices of these guns wear out quite frequently when zinc-rich primers are used, and hence they must be frequently replaced.
If you intend to apply a colored topcoat over the primer, you must first check with the coating vendor as to the type of topcoat with which the zinc-rich primer is compatible. For instance, in most cases you would not apply an alkyd enamel topcoat, whereas you might use an epoxy or polyurethane.
In contrast to the specified zinc-rich primer you might want to offer your customer a conventional corrosion resistant primer, such as MIL-P-23377, MIL-P-53022 or MIL-P-53030. Even though they might not provide the same degree of corrosion resistance to the substrate under corrosive conditions, they might nevertheless be adequate. MIL-P-23377 (in any of its formulation types) contains chromates, whereas MIL-P-53022 and MIL-P-53030 are chromate-free. The chromate-containing primer is considered to provide better corrosion protection than the chromate-free primers, but depending on the nature of the substrate (steel, aluminum, etc.,) and the degree of corrosivity of the environmental this might not be important.
These primers can be applied over clean bare metal or over previously applied paint. As with all coating applications, surface preparation is critical, and you should read the instructions that come with the primer. In any case, conventional primers are more tolerant to the surface condition that is the zinc-rich. Also, you have a greater choice of topcoat resin that you can apply.
In summary, zinc-rich primers are excellent for corrosion protection, especially in corrosive environments, but they are generally intended for steel structures and thick metal plates.