Research on blasting media turned up some interesting information on blasting thin sheet metal. Of course there are several factors that can have an affect on whatever media is chosen: The air pressure used for blasting, distance of the nozzle from the surface, angle of the nozzle to the surface, size of the nozzle, size of the media (fine, med, course), the quality and volume of air (proper CFM of clean dry air), and the amount of media allowed in the air stream.
Assuming that all those factors are correct, then it makes sense that the media should be just hard enough to make the anchor profile or etch pattern you want on the surface of the work piece for excellent paint adhesion. To measure the metal's hardness there are a few different hardness test scales to use, but the one used for blasting media is the Mohs scale, and it can also be used for metal. The hardness is measured on a scale from 1-10, 1 being talc powder and 10 being diamond--which is the hardest mineral.
|Mohs Hardness||Abrasive, substance or mineral|
|1.5||tin, lead, graphite|
|2.5–3||gold, silver, aluminum, zinc|
|5||apatite (tooth enamel), zirconium|
|5.5||molybdenum, glass, cobalt|
|6–7||silicon, opal, silica sand|
|7.5–8||emerald, hardened steel, tungsten|
Low carbon steel sheet metal is about 4.5-5 on the Mohs scale, so I want to find an abrasive that is a little harder than 5, and crushed glass rates a 5.5 on the scale, so that is excellent for rust removal and making a scratch profile for paint adhesion on low carbon steel. If you notice, glass beads are also 5.5, but the difference is in the shape. Crushed glass has a sharp angular shape, while glass beads are round and that makes them much less abrasive. Glass beads are used for peening or polishing the metal clean, and they are excellent for plating preparation, but not for metal removal, which is necessary for making the scratch profile needed for paint adhesion.
There are different grades of steel hardness, such as high strength steel which is used on later model car panels and they can go as high as 7.5-8 on the Mohs scale, but 4-4.5 will work good for cars prior to the 90's. In later years they added more carbon to make the steel harder, which is what gives it the higher strength. But increasing the amount of carbon was a gradual process over the years, so for those later model cars I would start with crushed glass which will probably work, and if necessary just go up the scale with harder media until you get one that will make a profile on the metal. You just don't want to go with a harder media than necessary, to avoid the distortion associated with blasting.
ANY media harder than steel carries the risk of panel warpage.
Benefits of Crushed Glass
- Crushed Glass will leave a clean metal surface.
- Leaves an excellent profile for paint adhesion.
- Glass leaves a clean white surface with very low particle embedding.
- Weighs less than most abrasives.
- SAFE - no free silica, non toxic, dust is classified by OSHA/NIOSH as "nuisance" dust.
- Much less dust than coal slag blasting media, which is very helpful in the blast cabinet.
- Mohs Hardness of 5.5
- Superior rust-back performance when compared with mineral/slag abrasives.
- Contains no highly toxic beryllium, like mineral slags do.
- Uniform density.
- 100% recycled.
Blasting with crushed glass is different from slags and other heavy abrasives, you must cut back on the media in the air stream for best results.
Start by purging your air line so it is abrasive free. Then slowly open the feed on the blast pot until you can see the abrasive in the air stream. Then slowly close the feed until you can no longer see the abrasive in the air stream. This is what you want. You may have to play with it some to get tuned in.
Blast at about 90 psi, or less. Higher pressure will be counterproductive due to how light glass abrasive is. You will simply shatter the glass rather than allow it to work for you.
Crushed glass usually comes in 3 sizes. Fine, med and coarse. You want to use med 40/70, its a perfect grit for paint and body work. Coarse is real heavy for car frames, bridges, etc. Fine is like baby powder, will remove paint - one coat at a time.
With 7.5 HP 24 CFM compressor I use 50-90 PSI, depending on the panel.
Sufficient CFM and clean dry air is essential for good results, and a compressor with less than 25 CFM of air will work best with 1/8 inch nozzle - just watch it for wear.
- #2040 Medium Coarse. This blend of grades is great frames MIL Profile: 2.5+ on steel surfaces.
- #40/70 Medium. Used for auto paint stripping and rust removal. (45-50 PSI when blasting low crown or flat panels like a hood, or thinner metal such as for autos from the late 90's-up) MIL Profile: 1.0 to 2.0 on steel surfaces.
- #70/100: Fine. Great on softer metals and substrates. Excellent for etching and de-greasing on aluminum. Works as a good substitute for soda blasting. MIL Profile: 0.5 to 1.0 on steel surfaces.
The biggest supplier of crushed glass is New Age Blast Media, they have several plants across the US, and a lot of distributors to save on shipping charges. Just give them a call to find a local supplier in your area. It should be around $8 per 50 lb bag.
Tractor supply has Coal Slag (Black Beauty) for about the same price, and it is 7-7.5 hardness.