I posted a thread on another forum on how I did a quick and easy re-paint of my Camry alloys, and received an interesting reply from a member who cautioned against sandblasting alloys because it would cause structural damage:

"What happens is the molecular structure of the alloy gets weakened from the sandblasting and can lead to catastrophic wheel failure. It can't be visually detected, but under a very high power electron microscope - the damage is evident in the metal, and the strength becomes compromised beyond the surface."

Since I was planning to do the same refinishing on my '86 560SL wheels, I was wondering if anyone here had heard of this before, or would care to venture an opinion? Here's a link to my thread http://www.toyotanation.com/forum/showthread.php?t=293160

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1986 560SL
2002 Toyota Camry
1993 Lexus

There are a lot of different media you can use to blast them. Maybe something softer would do better.

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" We have nothing to fear but the main stream media itself . . . ."- Adapted from Franklin D Roosevelt for the 21st century

OBK #55

1998 Lincoln Continental - Sold
Max 1984 300TD 285,000 miles - Sold
The Dee8gonator 1987 560SEC 196,000 miles - Sold
Orgasmatron - 2006 CLS500 90,000 miles
2002 C320 Wagon 122,000 miles
2016 AMG GTS 12,000 miles

I know that there are companies that "sand" blast with all sorts of media like pecan shells, plastic pellets, etc. depending upon the surface being cleaned.

__________________
" We have nothing to fear but the main stream media itself . . . ."- Adapted from Franklin D Roosevelt for the 21st century

OBK #55

1998 Lincoln Continental - Sold
Max 1984 300TD 285,000 miles - Sold
The Dee8gonator 1987 560SEC 196,000 miles - Sold
Orgasmatron - 2006 CLS500 90,000 miles
2002 C320 Wagon 122,000 miles
2016 AMG GTS 12,000 miles

I've sand blasted thousands of aluminum wheels without problem. DO NOT sand blast magnesium wheels. However, to minimize prep work after for paint, I'd try to use something softer.
Or spray with paint stripper and let it sit, wash off the peeling paint THEN give it a quick blast to take off the remaining coats.
Use a heavy duty pot blaster. A pistol type blaster would take forever and wear the surface unevenly. After blasting, the wheels will have to be taken down to 320 grit for paint. Sandblasted metal would take like 10-15 coats to cover the rough texture. Don't forget your Acid Etch primer it the paint will peel.

__________________
RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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FACEBOOK: PANZER450

Thanks for weighing in on this. Is there something about magnesium that makes the pitting more dangerous, along the lines of the explanation posted by babymog? I would speculate that magnesium is a harder, more brittle metal than aluminum, and cracks more readily... And if this is true, where do Mercedes alloys fit? Those small oval 'holes' around the edges would clean up nicely in seconds with a blast of fine sand (#00), a major PIA to do manually.

__________________
1986 560SL
2002 Toyota Camry
1993 Lexus

What's a pot blaster?

__________________
" We have nothing to fear but the main stream media itself . . . ."- Adapted from Franklin D Roosevelt for the 21st century

OBK #55

1998 Lincoln Continental - Sold
Max 1984 300TD 285,000 miles - Sold
The Dee8gonator 1987 560SEC 196,000 miles - Sold
Orgasmatron - 2006 CLS500 90,000 miles
2002 C320 Wagon 122,000 miles
2016 AMG GTS 12,000 miles

I assume he is referring to one of these, which is like what I used.


http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=40925

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1986 560SL
2002 Toyota Camry
1993 Lexus

yeah, that's it! there are bigger industrial ones like what I used in the body shop that will blast a wheel in a few seconds, if the OE finish is still on it.

__________________
RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

____________________________
FACEBOOK: PANZER450

That looks like fun. I had some friends back in the seventies who made sand-blasted, redwood signs using one of those.

__________________
" We have nothing to fear but the main stream media itself . . . ."- Adapted from Franklin D Roosevelt for the 21st century

OBK #55

1998 Lincoln Continental - Sold
Max 1984 300TD 285,000 miles - Sold
The Dee8gonator 1987 560SEC 196,000 miles - Sold
Orgasmatron - 2006 CLS500 90,000 miles
2002 C320 Wagon 122,000 miles
2016 AMG GTS 12,000 miles

I suspect that what the poster is cautioning about / worried about, is stress-risers created by the pits in the surface. Although it is a concern in aircraft parts etc., in the wheel surface it has a relatively minor effect on the strength of the wheel.

It is my opinion, that it is better to use a chemical stripper designed for the metal of the wheel (for an alkali metal if mag/al alloy as most are), less likeliood of damage to the wheel.

What you want to avoid, is removing metal with the paint. There are many different types of media from baking soda and walnut shells to silica sand and more. The best choice is one harder than the material being removed (paint), and softer than the base material (wheel), then only heat can damage the wheel surface.

If using sand, you do risk losing metal, and leaving areas sharp, both of which decrease the strength of the wheel and sometimes in critical areas. Even pits in the surface can be the beginning of a crack in highly-stressed areas, polishing and smoothing the metal will actually make it stronger (some parts are tumbled in ceramic or metal media to peen the surface and make the parts smoother and stronger).

What a "stress riser" is in simple terms, is creating a sharp corner, a pit, or other point where any strain/deflection (there is always some, even if only millionths of an inch per rotation) is concentrated because of the sharp corner or weak point. This will concentrate any deflection and stress on that point, eventually leading to embrittlement/failure at that point, a crack from which can propogate across the wheel. When a part is stressed and otherwise properly designed/manufactured, it only takes a tiny defect over many stress-cycles to create a failure.

So it is possible to weaken the wheel with sandblasting, but careful media blasting and polishing afterward can be safe.

__________________

Gone to the dark side

- Jeff

I'm very appreciative of your detailed explanation. Everything makes sense, from a scientific perspective, but the practical application suggests to me that what I have already done is acceptable.

The blaster was 1/3 full of some old recycled sand that I had swept up, strained, and returned to the hopper for 1 more use. It started out as #0. I continued to fill the hopper for this job with fresh #00. As I described in the link to the Toyota wheels I initially posted, each wheel was spotless after no more than 45 seconds. A lot of the original paint was still on them, just dulled by the sand. My intent was to give the self-etching primer more 'bite', and to do the cleaning job as quickly and easily as possible, not to strip off the surface of the metal. When the job was done, and after giving the backside of the rims a quick scouring, the hopper was almost empty. As before, I collected and strained the used sand, much of which was now almost powder.

As PanzerSD noted, "one would have to be a REAL idiot to harm the integrity of a wheel by sand blasting." I assume this would be the aggresive application of #1 or #2 silica from a high pressure industrial type sandblaster at air pressures exceeding the 100psi of my 5 HP home garage compressor, which actually only puts out this pressure for 30 seconds or so. It requires a recovery time of about 2 minutes to bring it back up from 80lbs after the 45 seconds of blasting each wheel.

__________________
1986 560SL
2002 Toyota Camry
1993 Lexus

sand granules are sharp and irregularly shaped, and can crudely eat away at the metal - what you want to do is bead blast it, with glass beads

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)

I must have missed this post, ... makes me laugh when I recall my own attempt with a sand-blaster and my little 5hp compressor, how very frustrating to not have the CFM!!!

Used a friend's diesel compressor/trailer with full power-vented suit, mucho better.

__________________

Gone to the dark side

- Jeff

I believe one would have to be a REAL idiot to harm the integrity of a wheel by sand blasting.

a Question. wouldn't small particles impacting a metal surface have a similar effect to shot-peening?

__________________
RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

____________________________
FACEBOOK: PANZER450

Small sharp particles would leave small occlusions with sharp edges, shot-peening is typically larger softer-shaped media that is designed to leave a smoother surface.

__________________

Gone to the dark side

- Jeff