Mental or Metal Problem?
Hi Joker,
"Powdercoating is not that good for wheels unless they are steel."
Have you had a problem?
John
http://www.metalbashatorium.com/heat_treating_aluminium.htm
My background makes me scared of some processes unless I understand them better. Anyway most decent wheels are T6 Heat Treated. Most crappy ones are T4 at best. There is a table in my link about 2/3 of the way down for those wishing to skip the reading for now.
The powder-coating folks will tell you that everything is fine and they have been doing this for x-long. How many folks could afford a proper metallurgical analysis of a failed wheel?
And somewhere I recall reading that 949 Racing replaced a set of wheels 6UL for someone that had them powder-coated maybe Keith Tanner from Flying Miata for the Targa Miata he was building. The powder-coated wheels had to be cut with a saw.
But if you paint, here is another good read (author is credited at bottom):
Powder coat is an excellent extremely hard finish. The polyester powder coat is the most weather resistant, but it tends to chaulk over time with exposure to the elements. Epoxy powder coat does not weather well due to the propagation problem. The BEST primer is an epoxy primer, which is formulated to be softer and grip better than regular epoxy. Zinc chromate epoxy primer is the very best and you can add a little vinegar for adhesion if you wish. I usually use an intermediate primer on top of the epoxy primer to get better adhesion to whatever topcoat I use. Any will work. The 'self-etching primer' is for painting bright trim flat black...popular in the 70's. Painters prefer the three coat system as it is most forgiving and can be overcoated within 5 minutes if you wish. Not so with other systems. It takes a little expertise to recoat a three coat system, but it can be done within the first year. After that it should be taken down to bare metal.
Oldtimers familiar with older systems feel more comfortable with nitrocelulose lacquer (extremely soft but can be worked to super gloss, also very hard to get.) Acrylic lacquer is very forgiving and can be re-touched and re-touched until you get it perfect. Acrylic enamel is a quick and dirty solution with a gloss additive if you are a good gun man and follow procedures. Polyurethane is a tough and extremely durable finish, but difficult to repaint. Not a good choice for topside applications.
In short, the three part system is the best for the buck. It resists scratches, can be buffed right away, holds the shine forever and is quite weather resistant. Disadvantage: If your paint is older than three years, you need to take it down to bare metal with a jitterbug and start from scratch. Do not try to sandblast the body as it will most likely distort the sheetmetal very badly.
The best quality and most durable paints on the market are DuPont. DuPont weathers better, buffs up better and holds the shine better than any other brand of paint.
Nitrocelulose lacquer is the best for absolute perfection and depth, but scratches very easily. 14-20 coats is typical. Typically used on top line show cars that never saw the light of day. Paint looks 10 feet deep. New clearcoats can approach this appearance, but can not match the beauty.
Acrylic lacquer is easy to work over and over and is still available. 7 coats is typical. Harder than nitro, but still scratches and is not very durable. My second choice for 50's and 60's cars done in your garage.
Straight enamel takes an expert, takes a long time to dry and doesn't buff up for years. Impossible to re-touch without doing the whole panel. Great for machinery.
Acrylic enamel is quick and dirty and looks great, doesn't scratch easily, can be buffed in due time (couple weeks). One light coat and one heavy coat over primer. Fairly tough. Not used much anymore. Best choice for engine enamel as it has the best heat resistance of one part paints. Repainting causes it to wrinkle up. (wrinkle paint)
Two-part Polyurethane is very tough and resiliant, beautiful high gloss with no effort, can be buffed, but impossible to touch up or recoat without doing the whole panel. Does not dissolve from solvent contact.. Best choice for under the car good looking high gloss applications where driving on the street is anticipated.
Epoxy two part is very hard and durable, and when coupled with a zinc epoxy primer will yield the best looking immediate undercarriage finish. (better than powdercoat) Excellent chemical resistance, does not dissolve from solvents. It will show wear and tear faster than polyurethane, so consider your intent.
Powdercoat has come a very long way in the last year. Fantastic color choices and temperature resistances. We just built a new state of the art powdercoat line for TPIS. For specific applications powdercoat is the answer. Chemical and solvent resistant. There is no soft binder coat under powdercoat, so it chips to the metal.
Primers: There are many primers for many applications. They are not multi-purpose in the true sense of the word. A specific primer is needed to barrier between reactive paints, like Variseal. Best regular primer is a Dupont lacquer based filler sealer primer. Best Best primer is what is used for military applications: two-part zinc chromate epoxy primer with a few drops of vinegar to etch the metal, topped with a flash coat of Dupont filler sealer primer. ANY paint system can be applied over most primers. Exception: enamel primer, not used much anymore.
Keep this in mind: EVERYTHING has it's advantages, but they all have an Achillies heel also. You don't get something without giving up something. The promoters will tout the wonderful advantages of their products, but they fail to tell you that there are disadvantages about EVERYTHING. NO one thing is a perfect solution and you must choose the properties you desire, or combine processes. THIS is why we went with the three step paint system, because we could solve all the technical problems with a combo procedure. It just meant a little more work in prep and app.
Bob Knoss, Hoffman Technical Engineering Advisor (retired)
