I am doing faux painting on my laminate countertop. I need to paint the aluminum strips that are along the front and side edges. What can I do to prep the aluminum?
My instinct is to cover the strips with masking tape, then prime, paint, and seal like the rest. My husband is afraid the tape will fail and peel away. That would be tragic after all this work. My experience with masking tape is that it adheres so well that it's hard to remove.
It's a dilemma that's keeping me from moving forward on the project. By the way, the counters are already done and look great. It's only when I started to remove the blue tape protecting the aluminum strips that I realized I should have painted them, too. They make the counter look more like Formica; if they were painted, it would fool the eye more.
Granite counters never have aluminum edging; only old 50's kitchens with laminate counters do. Just trying to update a little on this rental apartment. I'd so appreciate any feedback. This is a great site and has helped me tremendously with ideas, but I haven't seen this problem addressed. Thanks.
By Kathy G from Brooklyn
Aluminum is one of the most popular architectural substrates because of its excellent corrosion resistance, yet it is one of the most difficult substrates to paint successfully. A naturally formed, invisible and microscopically thin aluminum oxide covers the surface and renders the bulk metal relatively inert to atmospheric corrosion but because of its inertness paints and coatings tend not to stick well. The trick to achieving good adhesion is to remove the oxide and immediately apply a passivation layer to prevent the oxide from forming again. This is easier said than done.
Industrial Painting of Aluminum
Architectural aluminum handrails, windows, wall siding, automotive wheels, and other products that are exposed to the outdoors must be pretreated using a complex process comprising of a series of chemicals (Figure 1). Stage 1 of the 5-7 stage process, an alkaline solution degreases the surface to remove grease and oil. At least one tap water rinse, Stage 2, follows to remove the carry-over contaminants. Stage 3 comprises of a strongly acidic solution (deoxidizer) that removes the tenacious oxide. The rinse stage is again followed by a water rinse (Stage 4). Stage 5 the conversion coating is applied to the aluminum. The most common trade names are Alodine® (by Henkel) and Iridite® (McDermid), while Chem Film is also used. The process is often referred to as alodining, chem filming, or iriditing. After applying the conversion coating the aluminum surface is rinsed and sealed in one or more water stages and where high quality is required the final rinse stages comprise of deionized or reverse osmosis (RO) water. Finally, the clean, pretreated aluminum enters an oven where the water is evaporated to dry the surface.
The aluminum is now ready for painting. The oven temperature should not go above 140°F (60°C) because the coating tends to degrade. Fortunately, the process chemicals preceding the oven are often heated so that when the parts leave the final rinse stage the metal heat sink promotes fast evaporation. In crevices, channels, etc., standing water should be blown off with dry, oil-free compressed air.
In the electroplating industry where the finish is often decorative, the conversion coating process often comprise up to 10 stages.
Conversion coatings for military and high-end commercial applications where corrosion resistance is critical must often comply with MIL-C-5541 Class 1A or Class 3 "Chemical Conversion Coatings on Aluminum and Aluminum Alloys." The Class 1A Specification requires the conversion coating to contain hexavalent chromate (Cr6+); however, because this chemical is a carcinogen, the industry has, during the past few years, largely implemented non-chromate coatings to pretreat aluminum.
There are several new products on the market, some of which are based on siloxane technology. These can be applied with fewer rinse stages, and many are dried-in-place. In other words, they do not need to be rinsed off before entering an oven.
Many different organic paints can be applied over pretreated aluminum. In the architectural industry where long-term exposure to the exterior environment is required, it is common to apply a polyvinylidene difluoride (PVDF) coating to sheet aluminum and extrusions. American Architectural Manufacturers Association AAMA 2605-05 "Voluntary Specification, Performance Requirements and Test Procedures for Superior Performing Organic Coatings on Aluminum Extrusions and Panels" and AAMA 620-02 "Voluntary Specifications for High Performance Organic Coatings on Coil Coated Architectural Aluminum Substrates" are two examples. For less severe applications, such as indoor exposure, it is common to apply epoxy or polyester coatings.
The military applies paints to aircraft, tanks, personnel carriers, and other materiel that are made of aluminum. For instance, in the Air Force it is common to apply a strontium chromate epoxy primer, such as MIL-P-23377 "Epoxy Coating, High Solids" followed by a polyurethane topcoat, such as MIL-C-83286 "Coating, Urethane, Aliphatic Isocyanate, for Aerospace Applications".
The army has different requirements and on exterior surfaces applies an epoxy primer, such as MIL-P-53022 "Primer, Epoxy Coating, Corrosion Inhibiting, Lead and Chromate Free" followed by one of two chemical agent resistant coatings (CARC), MIL-C-53039 "Coating, Aliphatic Polyurethane, Single Component, Chemical Agent Resistant" or MIL-DTL-64159 "Coating, Water Dispersible Aliphatic Polyurethane, Chemical Agent Resistant".
In the commercial sector, epoxy primers are often applied in conjunction with two-component polyurethane topcoats. Most paint manufacturers offer products for these markets.
It is also possible to apply baking enamels to pretreated aluminum. Some coatings are applied directly to the passivated surface, while others apply a primer followed by a color coat. Baking enamels usually cure at temperatures above 250°F, but they provide excellent durability and color fastness.
For more than 25 years, powder coatings have been applied to pretreated aluminum. In the majority of cases the powder coating, often an epoxy, polyester, TGIC, or polyurethane are applied directly to the pretreated surfaces without first applying a primer.
When aluminum sheets or extrusions are intended for exterior application, especially for exposure near the ocean, it is advisable to first apply a corrosion resistant primer. Most aluminum sheets and extrusions intended for handrails, benches, sign posts, etc., are powder coated in this manner. Unfortunately, in my experience, coating specifiers do not always tell the powder coating contractor that the finished article will be exposed outdoors and especially near the ocean. Soon after the aluminum sheets or extrusions have been installed corrosion commences and large pieces of the powder coating fall off. (See Figure 2) The exposed bare metal is now allowed to corrode.
Painting of Aluminum Boats and DIY Applications
Arguably, the most questions by homeowners and on-site painting contractors concern the painting of aluminum surfaces. This is truly problematic because there is no simple method for removing the natural oxide on aluminum. Do-it-yourself (DIY) enthusiasts and on-site painters cannot use the strong and often hazardous chemicals that can be controlled and applied in industrial situations.
Aluminum can be degreased using power washers that contain soap solutions followed by a tap water rinse. For small jobs, chemical degreasers can be applied by oil-free rags and sponges. Rinsing with water is also relatively easy. Although the surface is now free of oil and grease, the tough, invisible oxide is still in place and the chemical conversion coatings used in industry do not adhere.
It is possible to abrade the surface with non-metallic abrasive paper, or in the case of thick aluminum plates and extrusions one can abrasive blast the surfaces with non-metallic abrasives, such as grit and garnet, but this is time consuming and expensive. Moreover, the primer must be applied as soon as possible to prevent the natural oxide from forming once again. Abrading the surface, either manually with an abrasive paper or by blasting provides the surface with a profile as shown in Figures 3 and 4. Adhesion of the primer layer is now predominantly achieved by mechanical attachment rather than chemical bonding. Of the two adhesion mechanisms, chemical bonding is usually considered to be superior, but the best is a combination of the two.
If you need to abrasive blast the aluminum, do not use steel shot as some of the abrasive will lodge within the aluminum and you can expect rust, followed by rust staining to form. This might not lead to any significant paint failure, but at the least, it will mar the surface finish.
An old standby, the vinyl wash primer to DOD-P-15328 "Primer (Wash), Pretreatment (Formula No. 117 for Metals)" was originally established during or soon after WWII and is still in use today. This two-component primer is water thin and contains so much isopropyl alcohol that most state air pollution agencies no longer allow its use. For instance, this high volatile organic compound (VOC) product > 6.5 lbs/gal is banned in California; yet historically, it was very useful for preparing aluminum substrates for painting.
In the early 1980s when light metal manufacturing for computers was booming in Silicon Valley, CA thousands of gallons of wash primer were used where the substrate could not be easily treated with a conversion coating. By the mid-1980s this product was banned and paint vendors tried to formulate waterborne equivalents. Although I have never tested them I believe that some products do meet the performance requirements of the DOD-P-15328 Specification.
Be aware than neither the solvent-borne nor waterborne wash primers can be applied over an existing conversion coating. Wash primers must be in direct contact with the aluminum substrate, else they will peel. In addition, the application data sheet should warn you that the dry film thickness must be in the range 0.3-0.5 mils (8-13 microns). At lower thicknesses the wash primer doesn't provide much corrosion protection, but above 0.5 mils (13 microns) it has a tendency to split and cause all the coatings above it to delaminate. '
In addition, I have had the unfortunate experience of applying a wash primer onto an already pretreated surface, followed by an epoxy full-bodied primer; when the coating system was tested in a salt spray cabinet extensive blistering took place between the wash primer and the epoxy. When the same test was performed on a system in which no wash primer was used, there was no blistering.
Some of the new clear siloxane chemistries can, apparently, be applied on-site and they have been reported to produce excellent corrosion resistance. An example is PreKote X-it® for which laboratory test results were published in [Ref], and Carpenter Chemicals recently published a paper on their Plaforization® coating [Ref]. Unfortunately, at the present time I am unaware if these products are sold to the consumer market. Recently, I experimented with PreKote X-it® on architectural handrails that had badly corroded, but it is too soon to report on the results. In preliminary laboratory tests, they passed over 1,000-hours salt spray [ASTM B-117, 'Standard Practice for Operating Salt Spray (Fog) Apparatus'o].
Typical Paint Failures
Based on my experience, the most common failures on handrails and windows is due to insufficient coating at cut ends and the sharp edges (90 degree bends) of the extrusions. Admittedly, it is difficult to deposit sufficient coating along the edges because paint tends to pull away. For instance, if a specification calls for a film thickness of 1.2 mils, it is likely that the measured film thickness will be less than 1.0 mil along the edges. Corrosion often commences when moisture permeates into the coating along the edge. Aluminum oxide (white corrosion product), starts to form under the paint coating and lifts it from the substrate. Over a period of time, often months rather than years, corrosion spreads under the paint film continuously undermining more of the paint. Eventually, the failure is so widespread that all or most of the paint delaminates.
Extrusions often have complex shapes, especially on products such as aluminum windows. The vulnerable areas for the early onset of corrosion are interior corners, crevices, 90 degree bends, or even more acute angles. When liquid paints and powder coatings are applied electrostatically these crevices or electrically shielded areas are known as Faraday Cages and paint coverage is generally poor to non-existent. Clearly, if these areas will be exposed to a corrosive environment you can expect corrosion to commence quite quickly. Once it starts it can progressively work its way under adjacent well applied films and cause corrosion and peeling.
Another common cause of paint failure is improper or insufficient surface preparation. As previously pointed out, it can be difficult to properly deposit a conversion coating on aluminum. With the gold-, amber-, or brown-colored chromate-containing conversion coatings, one can easily see where there are voids in the film, but with the new chromate-free clear coatings it is impossible to visually determine where the coating has been applied. In some fabrication facilities there is no consistent quality control (QC) prior to painting, so when the components arrive at the paint shop, the painters have no idea if the parts have been treated. Untreated parts tend to fail, especially if they are exposed to coastal or humid industrial environments. Therefore, especially for clear pretreatment coatings it is critical that QC be conducted before the parts get to the paint spray booth. Vendors of clear conversion coatings must provide a satisfactory QC method.
Because colored and clear pretreatment coatings are microscopically thin (1-3 microns in some cases) vendors recommend methods to strip the coatings from the substrates and measure the deposited weight in g/m2. Many in-house QC labs are capable of performing these tests. Since they rely on corrosive chemicals, a fume hood, four decimal electronic scale, and an oven. A well-trained technician can perform the tests.
The conversion coating process comprises many stages, as I have already explained. Unless rinsing between chemical baths is thorough, contaminants will be carried over from one tank to the next and the final coating will be suspect. When the rinse water from the final stage evaporates contaminants from one or more of the previous stages will remain on the surface. These are often soluble salts, such as chlorides and sulfates. In fact, some of the salts are hygroscopic and absorb moisture from the air. If you apply a paint system over a contaminated surface, even if the surface has been pretreated, the salts can absorb water under the paint film and corrosion can take place. Now you are back to the problem of progressive corrosion under the film.
Coating can embrittle over time and can cause microcracks. This is particularly possible when coil coatings are stretched during the profiling process. Similarly, coatings that are exposed to strong UV light (sunlight) degrade due to photo-oxidation, and this is particularly evident in the clear top coats of automotive finishes. Either of these two situations makes it easier for moisture to penetrate the coating and migrate to the aluminum substrate, leading to possible corrosion.
Identifying the root cause of paint failure on aluminum can be frustrating, because it is difficult to identify the presence of the microscopically thin pretreatment layer. Unless the laboratory analyst knows what to look for the task can be daunting. At least, if you know that the coating contains Cr6+, the analyst can test for it, but some of the new coatings are still proprietary and vendors will not tell you what to look for.
Sophisticated analytical techniques, such as scanning electron microscopy (SEM) followed by energy dispersive spectroscopy (EDS) are sometimes useful. Atomic force microscopy (AFM) is another technique for identifying the presence of a pretreatment coating, but if that is not helpful there are even more sophisticated methods that can be tried. The cost for conducting these tests usually increases with the sophistication of the instrument. Unless you are prepared to spend large sums of money on laboratory work, you might not be able to positively determine if a pretreatment coating was applied.
Successful application of paint to aluminum is no more difficult than applying the paint system to steel. However, the critical step is pretreatment. The conventional hexavalent chromate-containing pretreatment processes have been used for over 30 years and are easy to identify because of their yellow, amber, or brown color. With the advent of new clear chromate-free coatings, it is essentially impossible to determine if the coating has been applied at all, or if there are voids in an otherwise properly treated surface. Unfortunately, few reliable and simple QC tests can be conducted on a fabrication finishing line. Provided that the pretreatment coating completely covers the aluminum substrate it is likely that the paint coating system will perform well.
Where products, such as aluminum handrails and windows are exposed close to the ocean, such as on houses, vacation resorts, and hotels, the coating film thickness especially along cut ends and sharp 90 degree bends must be sufficient to prevent the ingress of moisture. The paint vendor should provide the minimum acceptable dry film thickness on the product data sheet. Figure 5 shows that at the 90 degree bend the film thickness is considerably less than away from the edge. Failing that, corrosion can commence where moisture penetrates the thin coating film. Thereafter, corrosion commences under the film and pushes even thicker films from the substrate. Figure 6 shows corrosion commencing at the outer 90 degree bend and spreading under the film. On the inside bend there is often little or no coating, and once again corrosion can commence and spread. Figure 7 is a micrograph of a powder coated handrail close to the ocean, where the corrosion spread under the film. Note how primer and topcoat are lifting from the surface.
Miter joints and cut ends are also a source for corrosion. In Figure 8 notice the corrosion that commenced at the cut ends and then spread away from the miter joint. Had a corrosion resistant sealer been applied to the miter joint, this problem could possibly have been avoided.
Architects and engineering specifiers must be particularly aware about specifying the minimum film thickness at the potential points of moisture intrusion.
In mild environments strong sunlight can play havoc with the coating resin, and for products located within 0.5 km from the ocean salt-laden moisture is the culprit. Since individual coating resins behave differently indoors versus outdoors, or in rural, semi-industrial, industrial, coastal, or chemical environments, architects and engineers must take the time to research the most appropriate resin system for the intended application. Do not forget QC. In most of the failure cases I have investigated the fabricator or coating contractor did not keep QC records of the pretreatment system, the coating weight of the conversion coating, or the dry film thickness of the primer and top coat. Without such records it is extremely difficult and costly to troubleshoot the root cause of the paint failure.
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