This section addresses consumer applications. Commercial and industrial painting involves specialized equipment.
The surface preparation is a function of the corrosion state of the item to be painted. This can range from almost no corrosion, which requires no initial preparation, to heavy rust which must be removed with a chisel or other tool.
The basic idea is to remove the rust that is to thick or to durable for a wire brush to remove. The only layer of rust that really counts is the one closest to the underlying metal. The idea is to remove the rust in the largest pieces possible, which reduces the time necessary for initial preparation. A number of tools are useful, including , pneumatic chisels and needle guns, and rotating wire brushes.
Initial preparation can be so time consuming that it isn’t worth the effort. If the corrosion has progressed beyond a certain point, the equipment has to be scrapped.
A small stainless steel wire brush is the best tool for final preparation. Once the metal surface is bare, i.e. their are nor rust deposits remaining on the metal, it is necessary to remove as much of the surface oxidation as possible. A small wire brush works very well, because the actual pressure on the surface can be 40 psi or more, which is sufficient to effectively remove the oxidation. The lower pressures associated with large brushes are nowhere near as effective in removing the oxide film.
Do not use a brass brush or a steel brush. Both brush materials will shed particles, and each particle will set up a galvanic corrosion cell.
The next step is to use a degreaser to remove any oil and grease that may be sitting on the metal surface. If oil and grease are present on the surface, the paint will stick to the oil and grease instead of the paint and the film will have little to no tensile, shear, or compressive strength. ICC offers an outstanding Degreasing Surface Cleaner. It is powerful, fast acting, and it is very compatible with rubber, paint, and plastic for this type of product. Some degreasers are very aggressive, and they will attack rubber, paint and plastic. Some degreasers will actually eat right through aluminum. Make sure to rinse off the degreased area very thoroughly. The residual degreaser will ruin the paint job and it will attack rubber, paint, and plastic.
Although it may appear contradictory, water is actually an excellent tool for preparing a surface for painting. Water is very effective at removing dirt and rust from surfaces. One way to see if a surface is ready to be painted is to wash down the area while agitating the surface with a wire brush. If you see red particles in the water leaving the surface, it is not ready to be primed. If you can agitate the surface and no rust enters the stream of water, the surface is ready to be painted.
Now its time for the primer coat
The primer coat is the coat of paint that actually prevents corrosion. The purpose of the finish coat is to protect the primer coat, not to control corrosion.
In the case of iron and steel, the primer of choice is iron oxide. If the iron oxide particles, which are actually small rust particles, are suspended in a hydrocarbon matrix, there will be no interaction between the oxygen in the air and the underlying metal. The iron oxide particles are inert, since they have already reacted with the oxygen in the atmosphere.
In the case of aluminum, the primer of choice is zinc chromate in a hydrocarbon matrix. Although chromium is a very expensive metal, it is very inexpensive as zinc chromate, since this is a natural form of both metals. Zinc chromate will not react with oxygen. The zinc chromate bonds quite readily to the aluminum, so this is a highly effective primer.
There is no small number of mistakes that can be made in applying the primer coat.
The biggest mistake is to apply primer over a surface with water, dirt, oil, or grease on it. The primer will adhere to the water, dirt, oil, or grease, and it will not adhere to the underlying metal. Applying primer on top of water, dirt, oil, or grease is a complete waste of time and money.
The next mistake is to apply primer to a surface that has not been properly prepared. In the case of steel, primers are fairly good at adhering to light surface rust. Since there is no difference between the iron oxide particles on the underlying metal and the iron oxides in a primer, the bond strength of the primer film is not adversely effected by a very thin layer of rust.
Try as you may, unless you have a sand blaster handy, you will never remove the surface rust. Even sandblasting suffers from a phenomenon known as flash corrosion. As soon as the abrasive exposes the iron particles, they react with the oxygen particles in the air stream used to transport the blasting media. The iron turns from a silver color to a gray color within milliseconds of exposure.
As long as the iron oxide is firmly bonded to the underlying metal surface, it is not a bad engineering practice to apply the primer over the light surface rust.
Aluminum is another situation entirely. Technically, aluminum doesn’t rust, it exfoliates. The aluminum particles actually separate from the underlying metal as a part of the corrosion process. Aluminum corrosion shows up as a light dust first. The dust is aluminum oxide particles. Eventually the process penetrates into the underlying metal, and eventually it looks very similar to iron corrosion. It is very important to remove all corrosion from the aluminum surface prior to priming, or the primer will not bond to the underlying metal.
Since aluminum has a significant electro-motive potential, it is highly susceptible to galvanic corrosion if steel, zinc, or copper are present with an electrolyte connecting the two metals.
Aluminum is highly over-rated as a corrosion resistant material. Many air conditioning conditioners are bare aluminum. They work very well at first, but over time the metal exfoliates, and eventually it corrodes. The aluminum fins end up being very mushy, and they do not reject heat very effectively. Once the condenser corrodes, it is only a matter of time before the compressor fails from overheating, even with pressure and temperature over limit interlocks.
Stainless steel is also highly over-rated as a corrosion resistant material. The only grade of stainless steel that is highly resistant to salt water corrosion is type 316. Most stainless nuts and bolts are type 302 because 316 is so hard that it is very difficult to machine. In addition, stainless steel likes to gall (weld itself together), which requires the consistent use of a thread lubricant .
A lot of paint jobs are ruined because the painter is in too big a hurry and the primer coat doesn’t dry completely. If this happens, the solvents in the primer are trapped underneath the finish coat. This ruins both coats of paint, since the paint was not designed to have un-evaporated solvents in it. Paint that is suffering from vapor entrapment never gets very hard. If paint can be scraped off with a fingernail a year after application, it is probably suffering from vapor entrapment.
If a primer coat is applied during inclement weather, it is highly prone to failure. The humidity can be 100% before, during, and after rain. 100% humidity means that every surface is a condensing surface. The film of water is not visible, but it can usually be felt. To paint over this film totally defeats the effectiveness of the primer. The water creates voids n the primer, so it is no longer impermeable to oxygen or water.
Once the primer has dried completely, it is time to proceed to the finish coat.
The finish coat is the next to the last layer of protection for metal. Paste wax or TC-11 is the final layer of protection.
Finish coats have a very high affinity for the primer, and they are very weather resistant. There is no such thing as weatherproof paint: a finish coat always fails, it is just a matter of how long it takes. If the finish coat is properly maintained, this can take decades and if conditions are “just right”, centuries.
In the case of equipment that is treated with TC-11, alkyd enamels are the paint of choice. They are durable, flexible, weather resistant, and 100% compatible with TC-11. This type of paint is often packaged as engine enamel. It dries quickly, and is an easy paint to work with.
Make a concerted effort to purchase the finish coat form the same manufacturer that provided the primer. Some finish coats and primers are completely incompatible,
Do not apply paint of the humidity if over 90% or rain is predicted. The relative humidity goes up to 100% when it rains, which means that the dew point is the same as the air temperature. This means that the surface will have a film of water on it, even if the rain does not impinge directly upon the surface. he water ends up leaving a void in the paint, and the paint with this fault may fail more quickly that a paint job suffering from vapor entrapment.
Pay close attention to the temperature. Paint manufacturers provide a range of suitable temperatures. The normal range is 60 to 90 F. If the surface is to cold the solvent will not evaporate quickly enough. If the temperature is too high, the solvent will evaporate too quickly. Either phenomenon results in low film strength.
Let the primer dry completely before the finish coat is applied. If the primer is not completely dry, the paint will suffer from vapor entrapment, which means that solvents that are not supposed to be in the primer are in the primer. The result is that the primer never gets hard. If the paint rusts within a year of application or comes off easily with a fingernail, it is probably suffering from vapor entrapment.
It is not unusual to have both conditions at the same time in the construction industry, i.e. the temperature and the humidity are not within the acceptable limits. Contractors are usually in a big hurry to get the painting done, and they will paint in any condition.
Boats, automobiles, and aircraft have spectacular paint jobs when they leave the factory. The finish coat on a production automobile is an engineering and artistic masterpiece, and the company has to spend millions of dollars to have the equipment to do it properly. Ironically, most of the automotive paint jobs fail because the owner doesn’t want to spend ten dollars a year on paste wax and two hours working on their car.
It is a good practice to apply two coats of finish paint. This takes care of any missed spots, thin areas, and requires a minimal additional effort.
If you are applying the final coat of finish paint and you have taped off areas, remove the tape when the paint is wet. If you wait until the paint is dry, you will chip the paint when you remove the tape.
Because paint is a maintenance item, the term paint maintenance sounds like an oxymoron. It isn’t. The annual equipment losses that may be attributed to improper paint maintenance runs into the billions of dollars per year.
There are two choices for paint maintenance.
Once choice is to apply paste wax according to the manufacturer’s directions. Modern paste waxes are easy to apply, easy to remove, and they last from six months to a year.
If rust is already in place, paste wax will not slow down or reverse the corrosion process. Paste wax has no useful lubricating properties. Paste wax has no penetrating capabilities.
The other choice is to use TC-11. The fact that TC-11 maintains the paint film is not the primary function of TC-11. The primary function of TC-11 is to lubricate articulated mechanisms, which are impossible to paint. The fact that TC-11 protects paint, as well as other rubber and plastic, is a secondary but useful benefit.
TC-11 requires less than 10% of the time required to clean the surface, apply the wax, and buff the wax. TC-11 is less expensive than quality paste wax.
TC-11 is not recommended for maintaining automotive finishes. TC-11 is ideal for maintaining equipment finishes.