Zinc-rich Paints: The Right Stuff

Q. We are in the process of painting transformer radiators with zinc-rich paints. Are there any special tools, spray equipment or methods needed for applying this type of material?

A. There are several application methods and tools you can use when working with zinc-rich coating materials, depending on the size and scope of the transformers to be painted. Determining the best application methods depends on whether the transformers will be painted in a building or on-site.

But before we begin with the application portion of this question, there are a few things you need to consider first. While working with several large manufacturers of transformers, one common factor I've noticed is that the heat sink radiator portion of the transformer is usually welded into position before the entire unit is painted. This always presents a challenge when trying to figure out the best method for coating, since traditional spraying methods do not enable a spray gun to get between the cooling fins. It is my understanding that the heat generated in the transformer during operation requires that these heat sinks be welded into place.

Surface preparation should be performed before any painting operation. It is important to remove all oil, grease and other types of contaminants with degreasers or similar cleaning agents. Take special care on the welded areas to also remove the flux and welding spatter. Most coating manufacturers recommend that the surface be painted fairly quickly after it is cleaned and dried in order to prevent "flash rusting." Some suppliers offer a pretreat primer that helps to arrest the rusting and contamination process.

After preparing the transformer, it is time to prepare the coating. Typically, many of the zinc-rich coatings on the market require a catalyst. It is important to follow the manufacturers' recommendation on the proper ratios of resin to hardener because improper catalyzing can affect the pot life of your working window. Manufacturers usually suggest a window 20 to 30 minutes after the initial mixing to insure that the two mixed parts have fully incorporated/matured before application. During this time, the zinc particles will generally settle to the bottom and will need to be mixed before application. Occasionally stirring or agitating the material throughout the process will prevent the zinc from settling.

For application, you can use a brush or a proper short nap or foam roller to paint the surface, typically without the need for any reductions. The roller technique is particularly effective in getting between the radiator blades/fins mentioned earlier. The brush or roller technique will give you better transfer efficiency compared to spraying, but is more time consuming, so you will want to keep a close watch over your pot life time window. You may consider brushing or rolling the hard-to-reach and complex areas first, and do the larger surface areas with spraying techniques. With either the hand or spray or method, you will want insure you are applying the proper mil thickness of coating to the surface as specified by the transformer manufacturer. If you decide to spray the large areas, you would want to use an "airless" spray system with a 3,000- to 4,000-psi fluid delivery capability.

As for equipment, zinc-rich paint's density and viscosity requires high-pressure delivery (as mentioned above) and the right-sized tip. Your spray equipment or coating manufacturer can provide specific information on proper tip sizes. I can say from experience that too small of a tip will give you constant clogging, which will lead to frustrating starts and stops and will work against your pot life time window. Err on the side of larger tip size and re-regulated fluid pressures to ensure a constant production flow.

Concerning the pot life, you will want to always use the mixed/catalyzed material within the recommended pot life period. All too often, operators try to use the material right down to the last drop, but if it has begun to thicken or crosslink, it will essentially fall out of proper manufacturing specifications and not perform correctly, especially in regards to adhesion. Most coating manufacturers recommend a temperature/humidity threshold for application. Typically speaking, don't apply material in an environment below 10° to 15°C (50° to 60°F) or with a relative humidity of less than 90 percent. Generally speaking, don't paint outside if it is raining, foggy or misting. Because equipment is expensive, you will want to clean your lines before the pot life window expires. I have flushed lines many times throughout the day just to insure the material does not crosslink inside my equipment. This is particularly important with the spray gun tip, because if the material sets up in the tip it will be almost impossible to clean correctly. You may want to purchase some "square broaches" as well as additional spray gun tips. Alternate the tips throughout the day, cleaning each between changes.