Seeking Advice Concerning Magnesium Wheel Reconditioning

Cast magnesium builds up stresses while driving, and those stresses do not entirely come back out by themselves. They may also age-harden and harder usually means more brittle.  Stories abound of magnesium wheels without tires cracking during rack storage! The U.S Military developed an annealing process to stress-relieve such wheels on high performance fighter planes. Powder-coating if done properly will also stress-relieve magnesium wheels. The keys are to get the entire wheel up to 275F and keep it there for a while, followed by extremely slow cooling- overnight is best. This temp will cause the Argent Silver paint to turn tan, requiring repainting. And because zinc chromate primers are difficult to find today, aggressive blasting is not needed nor recommended. The silver paint is decorative; the chromate primer is vital!

Hi

 I'm sorry if I am spamming this comment a little but I would like to try to rally up as much support as possible so I don't want anyone that might be interested to miss it. I hope you'll agree it is for a good cause. I have the blessings of the forum administrator.

I have made a De Tomaso Pantera  web page, here's a link: 
 Longstone Tyres - De Tomaso Pantera page
Please let me know if you see any mistakes. Any pictures you have that might make it better please send them over.

 Importantly if you may be interested in a set of the 285/50R15 P7 tyres please drop me a note via the "contact us" link so we can keep your email on file and use that collection of email addresses to convince Pirelli to make these tyres.

This is my first post here and I joined to get some insight into the process of refinishing magnesium parts. I'm surprised to read the advice on campis here because most of what I've read elsewhere differs. Cromodora wheels (often found on Ferraris and Fiats) are also magnesium. Originally they were pretreated with a chromate solution made by Dow (who are out of the business now) then they were painted. As I understand it, if you blast or strip the wheels down past the Dow layer (a greenish/bronze colour) then you really need to pretreat it again. Henkel/ Bondrite make a Dow equivalent called Alodine but the chemicals are pure evil.  (Steve Kouracs in his mag wheel tutorial mistakenly lists Alodine 1200 - which is only for aluminium not magnesium)

I was under the impression Pantera’s came with aluminum valve stems, not rubber. 

Aluminum with rubber gasket/washer. 

Larry

Does anyone know of a source for the original style aluminum valve stems? Thanks. 

http://hallpantera.com/cgi-bin...a-inc&item=22468

I am not an expert on the subject. I never paid attention to mag wheels prior to coming here. Based on what I've read and the conversations I've had over the last 15 years, it is my understanding that paint applied directly to magnesium shall eventually bubble and flake-off. A chromate conversion coating is an electro-chemical applied coating we are all familiar with. Tools and fasteners are often finished with chromate. The chromate conversion coating adheres to magnesium and in turn provides a suitable surface finish for primer paint to adhere to. The chromate conversion coating also prevents oxidation, and it is somewhat decorative. But due to the electro-chemical manner in which it is applied it provides a thin hard coating which does not chip-off like paint.

I assume De Tomaso mag wheels had a chromate conversion coating applied prior to being primed with zinc-chromate primer.

Dow 19 was a product that could spot coat areas where the chromate conversion coating had been damaged (curb rash). It left a brownish/goldish coating on the exposed magnesium. If Dow 19 is no longer available I think I would contact Marvic in Italy and ask them what they advise using in its place … and what primers to use as well.

I've also run across information on the net claiming that magnesium can be anodized, and that primer paint will adhere afterwards to the anodized surface.

Thanks George, I have finally tracked down a pdf from the original manufacturer of magnesium (the UK company was called Elektron) at this link:  

Surface Treatments for Magnesium Alloys in Aerospace & Defense

It talks about first pretreating the magnesium casting with a chromate conversion or hard anodic coating. (Never paint directly on magnesium)

then priming with paint containing not less than 15% by weight of chromate pigment. This should be either strontium chromate or strontium chromate containing up to 10% of barium chromate. Compounds of mercury and lead must not be used in the primer.

They don't mention zinc chromate or etch priming though.

Hi ROSSNZWPI, thanks for bringing this up, 100% agree there is a dearth of  believable  info on Mg treatment options currently available for DIY use.  I have 2 sets of wheels needing treatment but given all the contradicting recommendations online I have mostly decided not to address these as DIY projects.  Should that plan change, however, I'd go with the PreKote treatment you noted.  Everything I've seen online suggests it's a good solution, even though it's remarkably inexpensive (especially compared to possible Henkel/Bondrite options!!), available in small/reasonable quantities, AND apparently free of big environmental concerns.

My understanding of the DOW numbers (1-25 I think?) is that they refer to chemical processes, rather than specific chemicals previously sold by DOW.  While maybe some of the chemicals involved may have been proprietary and now NLA (DOW left this business line long back), at least "Dow 7" treatment is a current, viable option for Magnesium conversion coating, and certainly there are others available from industrial coatings specialists.  I assume DOW 7 is both hazardous or environmentally unfriendly, and I'm curious what advantages this sort of 'old school' approaches may have over PreKote.  Clearly the technical approach differs (i.e., adhesion promoter vs. conversion coating) but it does sound like major players (e.g., USAF) are using PreKote prior to painting some Mg aircraft parts. 

Re. the metal valve stems, several vendors must carry them - I recently got a set from Steve Wilkinson.....never thought about the possibility that they could create a corrosion concern. I imagine the securing nut could damage the painted surface of a wheel, but the metal stems were used on many other period cars with Mg wheels (Cromodoras come to mind).  Along this same line, I'm not aware of problems associated with the Pantera's steel lug nut seats, which must have been pressed into the wheel at manufacture, so I'm trying not to sweat this potential complication...!