DIY Grit Blasting.

[Richard]

DIY Sand Blasting.
Grit blasting or more commonly known as “sand blasting” is one of the easiest ways of removing paint and surface rust from steel. It is a simple process where grit is propelled through a nozzle by compressed air and is “shot” at the surface where the paint or rust is to be removed. It can be done commercially but it is a very costly process with a high labour content, but it is also easily done DIY with the right equipment.
Now before you head out to your local landscaping supplier and purchase some washed sand to blast with there are a few things we need to look at.
Firstly sand is not to be used as the blasting medium, as it breaks down from being smashed against the metal being blasted, it creates a fine silica dust that can cause problems to the lungs. But in saying that, any blasting medium used for grit blasting will still require the operator to wear a suitable dust mask, not only will there be dust from the blasting medium breaking down, but also dust in the form of paint and rust.
As particles of the blasting medium will be flying off in all directions once they have hit the metal, it will be necessary to wear a long sleeve shirt, long pants and shoes. My grit blaster came with a cotton hood to cover your head which was OK for a couple of jobs but I soon replaced it with a more commercial type hood which made the job a lot easier.
Now that we have all of the gear organised, it’s time to have a look at what we are going to blast. All of the parts to be grit blasted will need to be clean and free of grease and oil. If you grit blast an item that is coated in grease what you are actually doing is forcing the grease into the tiny crevices of the metal. This will stop your painted finish from sticking, so if you’ve got grease or oil degrease the parts before blasting.
Painting the grit blasted parts may seem a bit ahead of myself here, but as the raw finish of the metal will start to corrode as soon as the blasting stops you will need to at least apply an undercoat to seal the metal once you have finished.
I have been using a small grit blasting cabinet for years, the only drawback with this type of grit blaster is that the cabinet is never big enough for the part you need to grit blast. The blasting cabinets are available in various sizes and mine will fit a car wheel rim but it is awkward to blast as it is hard to get all around it. For this reason I usually only use this for small parts, and because I use medium No 3 glass beads as the blasting medium, this gives me a very good finish which I can then either zinc plate or paint. The gun on this grit blaster has a ¼ inch ceramic nozzle, which needs to be replaced occasionally as the blasting medium wears the inside of the nozzle. I have used various compressors with the blasting cabinet, but I find that my 240v 3hp compressor works fine with it. If you use anything less you have to wait every minute or so for the compressor to catch up.
Last year I was very fortunate to be given a 20 gallon grit blaster by a member of a visiting car club to the National Caravan Museum. Now this unit is for the serious DIY grit blaster. It is basically a 20 gallon tank that you fill with blasting medium, the air hose pressurises the tank which feeds grit into the main line which is forced out of the nozzle by the air pressure. Now the ceramic nozzle on this unit is about 3/8 diameter which in turn requires a lot more air supply. To run this unit I use a 3 phase compressor which supplies more than enough air to run the blaster continually.
As I use this unit for larger parts which usually have a thicker paint coating or have heavier rust I use a 30/60 garnet which is a great all round blasting medium. As I only blast with this unit out in the open I cover the ground with a large tarp (6m x6m) and place the item to be blasted in the middle. This helps pick up around 80% of the garnet which I can then collect in the tarp, sift out any large particles, and which allows me to reuse the garnet another 2 or 3 times. The only drawback with this type of pressurised tank unit is that the blasting medium flow rate is hard to regulate accurately, so you may be using more grit than is required.
One final thing to watch out for with any type of dry grit blasting, as the particles smash against the metal this causes friction, and in thin sheet metal this friction can cause heat distortion. The easiest way to avoid this is to lower the operating pressure and to work in a small area of about 100 mm square, then move to another area eventually joining up all of these areas.

[Richard]

Photo Index.
Photo 1
Grit blasting cabinet.

Photo 2.
20 gallon tank grit blaster.

Photo 3.
My unit ready for blasting.

Photo 4.
25 kg bag of garnet.

Photo 5
A caravan covered in plastic ready to grit blast the A frame.

Photo 6
Any part I don’t want blasted, like this ID plate, I cover with “duct tape”.

Photo 7
Some of the reclaimed garnet which needs to be sieved before re-using.