Preparing Metals For Finishing
SAF Painting Guidelines
PREPARING Aluminum FOR FINISHING — Do’s and Don’ts
This article helps subcontractors, specifically glazers, in the commercial construction industry avoid problems prepping aluminum for finishing. Glaziers trained on glass often miss problems with finishing metals that can turn a profitable job into a disaster. Finishing is where several costly errors might occur.
Here are some guidelines that can help you prevent problems and save money.
GENERAL METAL Application GUIDELINES
Make sure you understand the metal specifications. If the drawings are so confusing you don’t understand them, call the architect who drew them and start asking questions. You can also get advice from a curtainwall consultant or your metal supplier.
Study the material compatibility of the sealants, flashings and steel anchors to be used with the metal system. Sealant manufacturers are the best source for researching the compatibility of their products with metals.
Flashings should not be used next to a dissimilar metal. For instance, if you put copper roofing next to an aluminum fascia with no protection, you’ll get corrosion because of the galvanic action between the two materials, and premature failure will result. In addition, steel anchors must be primed to eliminate possible corrosion.
The Pretreatment chemicals for both the aluminum anodizing and painting process are so harsh they can destroy non-aluminum parts.
Distinguish what situations are appropriate for anodizing or painting. I don’t recommend anodizing for coastal locations — instead, painting is a better coating. However, don’t use painted coatings in an entry application-they are not abrasion-resistant enough for a door. Some powder coatings are hard enough for entrances, but anodizing is always safe.
Determine that the metal assembly to be finished has adequate drainage holes to prevent entrapment of the chemical solutions used for pretreatment. If they leak out later, the chemicals can damage the metal’s finish. Inks and dyes in packaging materials are another potential cause of staining.
Remove hardware or accessories before sending your part to be finished. To paint or anodize doors, the finisher must strip the hardware from the doors-most likely at an additional charge. I suggest that the assembly’s hardware remain unpainted. Although touch-up paint may be used on the hardware, it generally will not match nor last as long as a factory-applied finish.
Have assemblies finished after fabrication. Though doing so may not be practical for large assemblies, the result is a cleaner appearance. I especially recommend this if you’re going to be doing any welding or brake forming. Both painting and anodizing present special requirements for companies working with finishers. What follows are tips specific to either painting or anodizing.
Aluminum PAINTING GUIDELINES
Send a purchase order as far in advance of the metal’s arrival as possible. Often the finisher will have to special order paint, and delivery can take three to four weeks.
With each purchase order, include shop drawings of your material so finishers can determine what surfaces need painting and, if necessary, can use to identify your aluminum. Because many finishers charge by the square foot, you will save money by noting on the drawings the exposed areas requiring paint.
Remove all non-aluminum parts from any assemblies you need painted before shipment. Painting pretreatment may damage steel and plastic. Keep in mind a curing oven operates at temperatures as high as 475 F.
Because of the high temperature used to cure paint, flat aluminum sheets may warp in the oven as the metal expands and contracts. If flatness is critical, have finishers paint the metal after fabrication. Curved extrusions can change shape, so I recommend welding a temporary support rod across the base of an arch before painting.
Ask your finishers what is the maximum weight they can handle. Extra charges may be levied over a certain size or weight.
Remember the price of painting partially depends on the price of paint, which varies from color to color. Let your finishers know if you need a custom color so they can give you an exact quote.
Be sure that specialty items, such as curved arches, assemblies, or formed metal, are packaged soundly.
ALUMINUM ANODIZING GUIDELINES
Always check the alloy to be sure it’s anodizing quality (AQ). Try to get all your metal from the same lot to reduce color variation caused by the aluminum’s metallurgical composition. Die-castings are difficult to anodize because of the porous nature of the surface, even if the alloy is anodizing quality.
Because anodizing is only as good as the metal you use, it will not hide scratches or water stains.
Assemblies must have drainage holes to prevent solution entrapment. Top holes admit air, while bottom holes drain. Anodizing chemicals can leak through even the tightest weld joints.
Assemblies must not include non-aluminum materials. Be sure to select the correct alloy welding rod — we recommend 5356. The worst choice is 4043 since it will turn a smutty black.
For more information see our article about color variation.