Media Blasting and Air Compressors Explained

First off I believe that I must tell the reader a bit about myself and how I came to learn what I am about to place on paper.  In 1980, I applied for a got a Federal Firearms License as a way to support my love and hobby of firearms.  This allowed me to legally buy and sell firearms across the state line and to ship and receive firearms across the state line.

Soon my love and license turned into a nearly full-time business of selling and repairing firearms out of my home.  This also led me to get involved with another gentleman, also a FFL holder, and a business venture of applying a Teflon based coating on firearms as a weatherproof alternative to bluing.  This venture grew over the course of five years to the point that it was no longer fun.  It took my entire life and my wife's also, so we decided that it was time to stop.

The time that I had available led me back into LBCs (Little British Cars) and to the sharing of my experiences here. This information is also applicable to any kind of classic car or truck restoration.

The Types of Blasting Media

When someone speaks of media blasting, he can be talking about several medias.

Below is a partial list along with a bit of info about each.

Sand

Commonly know as sandblasting.  Sand, or silica sand, comes in several grits sizes and is the most common of the medias.  It is available nearly everywhere from building supply stores and hardware stores to specialty dealers.  It is cheap and easily handled in 50 or 100 pound bags.  It is used only once and is discarded after use.

In times past used sandblasting sand often was just left on the ground at the site that it was used.  Now OSHA and EPA have gotten involved and declared that used sandblast sand is a hazardous chemical because of the bits of paint and metal that is with it and must be disposed of in a fitting manner.  Sand is also rather soft as blast media goes so it breaks up readily.  When it breaks up it forms silica dust that can be inhaled.  This can lead to a very serious disease called 'silicosis'.  Silicosis is similar to black lung in the fact that the dust gets into the lungs and stays there forever thus reducing you ability to breath.

Sand can be used in an enclosed cabinet however the fact that it breaks up so readily makes seeing hard and quickly reduces your blasting ability as particles get smaller.  As a general rule stay away from sand in an enclosed cabinet and use outside if you can.

Sandblasting a classic car body outdoors in preparation for bodywork and paint

Slag

Slag has become a cheap alternative to sand.  The slag is created as a waste product in the production of iron.  It is like glass powder.  It is available under a variety of names, one of which is A Black Beauty.  I tried this stuff when my supplier gave me a couple of free bags to try.  When I was working on guns I was blasting a lot of small parts, some of which were case hardened.

I found that this media would work OK for some of my jobs but would not even touch the hardened steel parts.  I was searching for a media that would provide an uniform surface finish on all the materials used in construction of firearms.  This was not it!  Slag was available in several grits, and was cheap, less that $2 a bag.  The down side was that it broke up in use much like sand so it required keeping several hundred pounds on hand at all times and then dumping the old used grit and replacing, both of which required more time than I had.

In addition to the fact that it was not hard enough to provide the finish that I required it was not worth my time to mess with.  It would work fine in an outside blasting situation where you did not reuse the grit, and like sand, many people use it for this.  It too, becomes hazardous waste upon use.

Aluminum Oxide

Aluminum Oxide (Alox) was my savior.  Alox is used in making abrasive paper and cloth so you know that it is hard enough to cut metal.  Alox was hard enough to affect the hardened steel gun parts.  It also was hard enough that it did not break up in use like the other medias did.

The problem here is that it is not cheap!  So far I have been talking about media that cost less than 2 bucks a bag.  Alox was costing me closer to $20 for a 50# bag.  That was the down side.  The up side is that it made little dust, and seemed to last forever.  It was not as readily available as the others but I had a good supplier that would always keep a couple hundred pounds in stock for me but I had to drive 75 miles to get it.  Shipping via UPS was expensive.

This is the media that I settled on for most of my work.  It comes in several grits.  I used 60#, which gave a rather coarse finish that was begging for paint.

Silicone Carbide

Silicone carbide is an even harder media than Alox.  It will attack almost anything.  It is also at least twice the cost and a lot harder to find.  I had a small amount as a sample but decided that the benefits did not out weigh the extra cost, so I never did any serous testing with it.

Glass Beads

Glass beads are available in a multitude of grades.  Glass has a couple of disadvantages.  One is the fact that it is not hard enough to attack all materials.  Another is the fact that the beads are rather fragile and should be used with less than 80# air pressure to keep them from breaking up prematurely.

The beads are perfectly round, and being perfectly round they leave round indentions in the metal that they are used on.  This make a neat finish because of the way that light is reflected off the surface.  Coarse beads will create a sort of pearl finish that appears to be a foot thick.  The finer the grit, the smaller the spherical indentions, the less of this effect you have.  Also, the fact that you have rounded indentions gives paint a less desirable surface to bind to as compared to the other medias mentioned above.

In my shop now I maintain one cabinet with coarse glass beads for my general purpose blaster.  When I rebuild an engine all the internal parts are blasted in this cabinet.  It made the rods and rockers come out BEAUTIFUL!  Glass beads are also available in combination with Alox for a more general purpose media.  One drawback with beads is the fact the will break up rapidly if you use too much pressure.  I try to maintain around 60# in my glass bead cabinet.

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Other Grit

There are other medias that I have no experience with.  Just to make you aware of their availability I will list them.

• Plastic beads
• Steel grit
• Nut shells
• Brass or aluminum shot
• Ceramic shot
• Broken glass

All of these have their applications.  Some would be cleaning paint off of metal without affecting the metal.  Another would be removing rust without affecting the blast metal.  Naturally, the softer the base metal the softer the grit need to be.

Not your average sandblasting technician...

Media Blasting Equipment

Basically blasting equipment can be broken up into two main systems and then subsystems under those.

Pressure Blasting

This is the familiar pressure pot that you see in use sandblasting large equipment like tanks and bridges and such.  This is mostly used outside in the open but there are a couple of setups that I have seen that used a self enclosed and circulating box with a pressure blaster.  I have also seen self enclosed rooms where a operator gets inside and walks around while blasting a object while the grit used falls though the floor to be reused over and over.  Neat, but it does not have anything to do with this discussion except for the fact that it would be neat to use a setup like this on a classic car!

Siphon Blasting

This is the system that you are most likely to run across and use in our hobby.  There are pots for use with a siphon blasting gun for exterior use but they are SLOW to try to do a whole car with.  Most cabinets are of the siphon variety.  Both of my setup are of this type.

Blasting Cabinets

Cabinets are available in nearly any size from shoebox size to ones big enough to put an entire fender in.  When it come to size you cannot have a cabinet that is too big as there is always something larger than your cabinet that you need to blast!  Cabinets can be homemade, as my first one was, or factory made.  In hindsight, I would never attempt to make another one myself because there are too many nice cabinets available to justify spending your time trying to design something that you will be unhappy with later.  That said, I will tell you what makes a good cabinet, in my opinion.

First, a cabinet has to be air tight.  You will need to run you cabinet under a slight vacuum in order to control the dust created within.  If you disregard this you will have one big mess on your hands.  Think about it - you will be adding air to a box at a rate of 10-15 scfm.  That air has to go somewhere after it get there.  If you have not provided for this air and dust will emerge from ever crack that you knew about and a bunch that you didn't.  Believe me, I've been there!

Second, your box will have to have a funnel shaped bottom to be able to feed your blaster.  Just a little slope won't do.  Sides need to be at least 45 degrees.  The only time that grit will flow like water is when you have a hole and you are trying to keep the grit inside-then it will go all over the floor.  Your funnel needs to have an opening in the bottom so that you can drain the grit out when you want to change it out.  This opening also needs to be high enough above the ground so that you can get a pail or box under it to catch the grit.

Third, you need a window to be able to see what you are doing through.  This window needs to be covered with a layer of plastic to protect the glass from the grit.  My first box used just a layer of window glass and in a short period of time it was ruined.  I started cutting up 3-liter coke bottle and using this to cover the glass.  This worked OK but I could not drink enough cokes to keep up!

Fourth, you need a source of light to see by.  Some of the commercial boxes have a flood lamp mounted inside.  This sucks because the grit gets up in the socket and destroys everything after a short period of time.  In addition to this there is always the likelihood of hitting the bulb and breaking it at the most inopportune time - been there, done that.  I now use a florescent trouble light mounted on the outside of my window.  It is out of the way and protected from the grit.  The ideal solution would be to have another window on the top of your box with a light behind it.  Next, you need access to the inside with your hands.  I took a pair of rubber gloves and had my wife sew them to a pair of jean legs.  It worked for a while but you will learn that the left glove always wears out first if you are right handed.  You unknowingly blast the dickens out of the thumb and forefinger while you are holding your target.  Naturally all this needs to be at a comfortable work height.  I spent eight hours a day in front of my cabinets.  More than five minutes of bending over will give me a backache big time!

Next, you have to have some method of dust collection.  I can hear what you are thinking - no, a shop vac will not do!  I went through three of them before I learned.  Most shop vac's use the air that you are sucking to pass through the motor and cool things off.  OK, think what is going to happen when you put a bunch of Alox in this air.  Goodbye brushes, goodbye bearings!  Yea, I know, the shop vac has a filter, but it ain't enough of a filter, and it only takes one small hole to let enough grit through to destroy a motor!  Buy a Dust collector that is made for the job and save yourself money in the long run or work out a deal to buy vac cleaner motors by the case from WW Grainger!  Been there, done that!

Blasting Cabinet in operation, keeping your media where it belongs

Guns aka Nozzles aka Heads

The last and most important item I saved for last - the gun/nozzle.  I will call it the head from here on out 'cuz head is easier to type and my fingers are getting tired.  There are good one available and there are cheap ones.  I have had both.  I will never buy another cheap one!  The head has to stand up to the flow of both the air and the grit.  It is basically an aspirator-air flowing through a venturi tube create a vacuum to suck the grit up from the bottom of the box.  The cheap one are made of some pot metal alloy.  The good ones look the same but they have a steel liner in the proper place to keep the grit from eating it up.  They both look the same on the outside.

There is a company called TP Tools & Equipment that specializes in blasting equipment.  They sell everything that you will need, order a catalog.  You may not thank me if you do it now but if you mess around trying to cut corners and do it later you will say  why didn't I do this sooner.  Their catalog is worth it even if you had to pay for it, which you don't.

Nozzles are available in three materials.  First is mild steel.  This is the cheapest and it reflects in the life if the nozzle.  Grit will eat it up in a hurry. If you are not careful it will destroy your head at the same time.  Next is ceramic.  Ceramic is harder and tougher than mild steel but is brittle.  Cost is naturally higher than steel.  Last, we have the Cadillac, 'er excuse, the Rolls of the nozzles.  It is made of tungsten.  This is the one you want if you are going to do much blasting.  It will last and last and last.  It is however, expensive.  I use nothing but tungsten!

More on Nozzles

Ok, if this has not been confusing enough already I am going to throw some more at you.  Nozzles come in three sizes.  The size needed depends on the air flow that you have available.  They are- 13/64", small, requires 4-8 scfm, 1/4", medium, requiring 10-15 scfm, and 5/16", large requiring 20-25scfm. 

This is where thing get critical.  If you do not have enough air flow you will not suck the grit up and through the head.  Too much air is not a problem but you are not using all of you capabilities.  In both of my cabinets I use medium nozzles.  I have two Campbell Hausfeld 5hp, two stage compressors with 80 gallon tanks.  Each compressor will put out about 12 scfm at 125#.  This is barely enough to get by running two cabinets.  In choosing a compressor you need to know how much air you will need.

Ideally, your compressor should put out 30-50% more flow at your required pressure than you need.  This will allow you compressor to cycle off in times of heavy use.  If you use more air that your compressor can provide your pressure will slowly drop to the point where you blasting will suffer.  The tank size will act as a buffer of sorts to lengthen your blasting time but is no substitute for enough compressor.  One thing that a larger tank will do is to help cool your air a bit to allow moisture to condense out while in the tank.  Blasting cannot tolerate ANY moisture without plugging things up.

If you must have a compressor with a small tank you can rig up a coil of 2" copper tubing in a five gallon bucket of water and then a water separator downstream to help.  You will also need a regulator/separator at the point of use to get the last bit of water out.  Down here in south LA where humidity runs 100% most of the time I have to go to extremes to deal with moisture.

One more thing: put your compressor outside, on a concrete pad under a roof, blow the tank down and check the oil level every day.

Conclusion

I hope that I have not left anything out.  I have worn out several compressors and blast cabinets and made a LOT of mistakes over the years.  Get the biggest and best compressor that you can afford and run it on 220VAC if at all possible.  As a kid, my Daddy used to tell me to get the very best tools that I could afford and take care of them and they would last me a lifetime.  It took me years to realize how smart those words were.  I think they are even more important in these times when so much of the equipment available is pure disposable crap.

I would be happy to address any questions or argue any of my beliefs if you care to, just scroll down and leave a comment!