By Liam Venter at FastBikeGear
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 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.
Because magnesium is so reactive, magnesium wheels are a different care and maintenance proposition from alloy wheels. 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 should be X-rayed by the manufacturer to check for any voids or anomalies in the castings. Some people would suggest that they should also be x-rayed 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 classified as 'sub optimal'.
We recently purchased a set of second hand very light weight cast magnesium wheels for a FastBikeGear project bike. the wheels were in pretty good condition although 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.
Plan A: 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....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 and refurbishes magnesium helicopter parts and I get put through to a South African guy who suggests I bring my wheels to look at and some beer to drink. To cut a long story short this extremely helpful and knowledgeable gentleman (South African air force certified 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 and confirmation that my wheels were originally chromated with the Dow 1 treatment.
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.
Step 1: 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. Other forms of blasting to remove paint are not recommended!
Step 2: 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 tips I got. The Dow 19 will also lightly chromate the surface as it cleans. You may see the surface turn slightly brown or gold.
Step 3: 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 richer brown or gold colour.
Step 4: Wash off the Dow 1 with clean tap water.
Step 5: Prior to painting, wipe off finger print oil, etc with a professional painters grade wax and grease remover (available from auto painting supply shops)
Step 6: Prime and seal with 2 part epoxy etch primer. Recommend Altex AltraBond 3094 CF - used and certified by the aerospace industry for priming magnesium parts and readily available in New Zealand. You can powder coat magnesium, but it is sub optimal. 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 anything else!"
Step 7: Paint with your favourite 2K top coat.
Step 8. Use only zinc plated fittings. Zinc is the closest metal to magnesium on the galvanic table which means it is the least reactive with it. It is imperative that any dissimilar metal that comes into contact with bare 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!
Hope this helps someone else.
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 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.
Because magnesium is so reactive, magnesium wheels are a different care and maintenance proposition from alloy wheels. 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 should be X-rayed by the manufacturer to check for any voids or anomalies in the castings. Some people would suggest that they should also be x-rayed 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 classified as 'sub optimal'.
We recently purchased a set of second hand very light weight cast magnesium wheels for a FastBikeGear project bike. the wheels were in pretty good condition although 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.
Plan A: 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....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 and refurbishes magnesium helicopter parts and I get put through to a South African guy who suggests I bring my wheels to look at and some beer to drink. To cut a long story short this extremely helpful and knowledgeable gentleman (South African air force certified 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 and confirmation that my wheels were originally chromated with the Dow 1 treatment.
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.
Step 1: 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. Other forms of blasting to remove paint are not recommended!
Step 2: 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 tips I got. The Dow 19 will also lightly chromate the surface as it cleans. You may see the surface turn slightly brown or gold.
Step 3: 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 richer brown or gold colour.
Step 4: Wash off the Dow 1 with clean tap water.
Step 5: Prior to painting, wipe off finger print oil, etc with a professional painters grade wax and grease remover (available from auto painting supply shops)
Step 6: Prime and seal with 2 part epoxy etch primer. Recommend Altex AltraBond 3094 CF - used and certified by the aerospace industry for priming magnesium parts and readily available in New Zealand. You can powder coat magnesium, but it is sub optimal. 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 anything else!"
Step 7: Paint with your favourite 2K top coat.
Step 8. Use only zinc plated fittings. Zinc is the closest metal to magnesium on the galvanic table which means it is the least reactive with it. It is imperative that any dissimilar metal that comes into contact with bare 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!
Hope this helps someone else.
