Refurbishing Magnesium wheels.
I recently purchased a set of second hand very light weight cast magnesium wheels for my Ducati 1998 F.E. SuperSport project.
The wheels came from Japan and were branded Duomo but are actually rebadged Marvic Penta 2 wheels designed to fit the later model fuel injected SuperSports.
Motorcycle wheels are made from either Aluminium or Magnesium. Aluminium and magnesium wheels can either be forged or cast. Forging results in stronger and more uniform material grain so forging allows manufacturers to make lighter wheels. The good news with Magnesium is that you can make wheels that are both lighter and stronger than aluminium wheels. For example my cast magnesium front wheel weighs 3kgs (with bearings fitted), which is a massive 2kgs lighter than the original OEM wheels. Magnesium because of its terrific strength/weight ratio is extensively used in the aerospace industry and is now making a small comeback for components on top of the range sports bikes.
Magnesium wheels are a different care and maintenance proposition from alloy wheels. Because magnesium is so reactive. Magnesium sits at the very top of the galvanic table, just above zinc. When it reacts (corrodes) with another metal it is unfortunately always the donor material!
Cast magnesium wheels are sometimes X-rayed by the manufacturer before shipping to check for any voids or anomalies in the castings. And some people would suggest that they should also be crack tested periodically during their life as well. Corrosion can track and worm-hole through magnesium and can go unnoticed under paint (particularly powder coat). Corrosion around dissimilar metals could in extreme cases cause things like brake discs and sprockets to part company with your wheels under the worst possible circumstances. In marketing terms this would be defined as sub optimal.
When I got my second hand cast magnesium wheels they had a few paint nicks in them from careless tyre fitting and also there were two isolated small spots where the paint was lifting. No problem.... visit some wheel painters and get them touched up...um only problem was that the ones I visited didn't have a clue on painting magnesium...umm we'll just sand them down, etch prime them and give them a lick of top coat....actually no you won't!
Plan B, jump on the Internet forums and get the answers on the best way to touch up magnesium.......only problem is that all the advice was contradictory, and most of the advice I subsequently discovered was well meaning...but very poorly informed...and many people reported unsatisfactory results.
Eventually I got some maintenance documents from Elektron in the UK who manufacture magnesium for use in the aerospace industry. I then found a company in N.Z. who were willing to mix up the pre-paint treatment recommended in the maintenance documents....two weeks later the secret potion arrived and I swabbed it on...and waited for the magnesium to acquire the predicted nice brown or gold coloured chromate finish,...and waited ...and waited ....and nothing happened! So back on the phone to the guys who mixed up the chemicals for me. In the ensuing very helpful conversation they mentioned the name of the only other company in New Zealand that they had ever made the treatment for. What the hell I thought I will be cheeky and give them a call.
So I call up this company who services magnesium helicopter parts and I get put through to a South African guy who suggests I bring my wheels around for him to look at and some beer to drink. To cut a long story short this extremely helpful and knowledgeable gentleman (South African air force trained engineer) gives me a practical demonstration of the entire pre-clean and re chromate treatment programme. Like all things it's simple when you can find someone with the expertise to tell you how. I also emailed Marvic who manufactured my wheels and got some very useful refurbishment information and advice from them and confirmation that my wheels were originally chromated with the Dow 1 treatment. Marvic were MOST insistent that I also crack test the wheels as they were several years old.
Here's what I think I have learned from these people and my experience.
Note, Dow 1 and Dow 19 referenced below are nasty chemicals, use in well vented areas and wear disposable gloves at all times.
Remove old paint. Do not attempt to remove paint from magnesium with any metallic implement, scourers or chemical type paint strippers. You can use plastic media blasting but this must be done gently at a low pressure as the surface of magnesium is relatively soft. The plastic blasting media used must be brand new as magnesium is very reactive with many other materials and contaminates that are likely to cause corrosion in the future can easily be embedded in the surface. You can also use Soda blasting but as I found out due to it's extremely gentle action it can take some time to remove baked on paint. All soda must be thoroughly removed after process. A key advantage of using soda blasting is that it does not peen the service so that you can crack test the wheels using a dye penetrant.
If you are repainting Magnesium wheels it is probably timely to crack test them using a dye penetrant A fluroscent dye penetrent is superior to the non fluroscent dyes. You can purchase the dye from engineering supply shops. You will need a ultraviolet light to inspect the wheels after applying the penetrent. Luckily you can purchase LED ultraviolet torches that are suitable for doing this cheaply now.
Clean bare magnesium with Dow 19 and a Faber Castell fibre glass tipped pen (available from arts supply shops). It's incredible how well these pens work compared to my previous attempt with a green scotchbright cleaning pad. Definitely one of the best bit of advice I got. The Dow 19 will also lightly chromate the surface as it cleans. You may see the surface turn slightly brown or gold. It is essential that you remove EVERY TRACE of corrosion before moving on to the next step.
Contrary to popular belief a chromate conversion coating is not an anti corrosion protection in it's self. What it does is provide a finish that a primer or powder coat can adhere to. If you don't chromate the magnesium before painting you can guarantee that your beautiful paint finish will lift off some time in the future. From what I have learned, If you find someone who says that they can paint un-chromated magnesium with an etch primer then you have just found someone it would be best not to trust your wheels to. There are several alternative processes to create a chromate conversion layer on magnesium but the best for touch ups is Dow 1. You just wipe the Dow 1 on and leave it on for 3 minutes. The Magnesium should at this point change to a rich brown or gold colour.
Wash off the Dow 1 with clean tap water.
Prior to painting, wipe off finger print oil, etc with a professional auto painters grade wax and grease remover (available from all auto painting supply shops)
Prime and seal with 2 part epoxy primer. An engineer mate of mine once looked at my race cars powder coated frame and said "powder coat is an excellent finish …..for indoor furniture but has no place on any motor vehicle!" There is some debate over whether it is better to use an etching or non etching primer. If you can find one with chromate in it so much the better. For health reasons most modern primers are chromate free. I used Altex AltraBond 3094 - which is a etch primer used and certified by the aerospace industry for priming magnesium parts and readily available in New Zealand. This paint used to contain chromates but is now only available now as a Chromate Free (CF) blend. If you are getting someone else to paint the wheels make sure you instruct them NOT TO SAND the chromate surface prior to painting.
Paint with your favourite 2K top coat.
Use only Zinc plated fittings. Do not use any stainless fittings near magnesium! It is imperative that any dissimilar metal that comes into contact with magnesium is coated with a suitable anti-seize jointing compound. Mastinox 6856K jointing compound is recommended. It is expensive but it is certified for use in the aerospace industry with magnesium. Do not use chromium or copper based anti-seize or jointing compound as it will react with the magnesium...um quite quickly as it turns out...don't ask!
Regularly wash and inspect your wheels for cracks and damage.