required air flow for small sandblaster

[Renze]

Hi,

i have some jobs to do, currently rebuilding a car tandem trailer. Usually the conservation of the workpiece suffers from the lack of time (patience) so the paint is blown over half rusted iron.

I'm just too lazy to remove rust from hard to reach spots with a wire brush or a screwdriver. Just rage over it with the wire brush in the 2300 watt Hitachi angle grinder and pour it on... obviously this is not the way it should be... :S

I was thinking of buying or making a sand blaster, but i dont know what air flow i need to keep blasting without running out of air and waiting 5 minutes for the kettle to build up pressure for every 30 seconds of sand blasting.. ???

Maybe i should find a Roots blower from a grain truck, at the local wreckyard ?? I'm sure that has the power and capacity to blast like h3ll and give my tractor its hands full on the PTO....

 

[Bird]

I think the question is too general for anyone to give a valid answer. If you look at sandblasters from Sears, Northern Tool, Harbor Freight, etc., you'll find they call for 6 to 25 CFM depending on nozzle. I've used a Sears sandblaster than only used a 1 qt. can like a paint spray gun and you don't have to worry too much about having enough air flow because you spend all your time refilling the can with sand. I also used to have a 100# capacity sandblaster and a 6 CFM compressor could not keep up at all.

 

[Renze]

Thank you bird, it gives me an idea. A tool trader that sold me a used band saw 2 or 3 months ago, also had blasting barrels for sale. He told me to go with 350 liter per minute. 1 cfm is 27 liter, so thats allmost 13 cfm
I do have a small air compressor with a 25 liter barrel that will do at least 6 cfm, but would this air compressor with a harbor freight or northern tool equivalent (chinese) blaster remove any substantial rust ?

I am not talking large areas here, but i do want to clean some serious rust, not just create a finish on automobile tinwork.

What kind of work do you do with your blaster and is it tedious ? Would you recommend your setup for someone who has limited patience ?
I wont use it that often, just when i need it so i dont want to invest large amounts of time (the PTO blower) or money (buying a high capacity workshop compressor) into it.

 

[Bird]

I bought the little 1 qt. sandblaster when I had a 1 hp compressor on a 20 gallon tank to do some work on an old truck. My Dad had bought a 1967 Chevrolet half ton, former telephone company line truck that had been sitting a long time in a pasture. He had all the running gear up to snuff, but the body was all doors, drawers, etc. and what I couldn't do with a sander, I sandblasted; inside nooks and crannies and drawer compartments. I spent a lot of evenings after work and weekends. I painted inside the drawers and doors with rattle cans, then a friend who had a body shop sprayed the exterior of the truck. Then Dad put all his fishing gear in it and had the biggest mobile tackle box in Port Aransas, TX.

And I bought the 100# rig to do just about what you're doing. I sandblasted the little 5' x 10' tiltbed trailer I had and painted it. One of my brothers bought a similar sized, but not tilt, trailer cheap simply because it was looking bad and the seller needed a few bucks, so I sandblasted and painted it, then set it out front and sold it.

Would I recommend it for someone with limited patience? Absolutely not, and that includes me. I did not do a particularly good job either time and I won't do it again with a small rig.

 

[Renze]

I was just thinking: i have an old 1.7 liter gas motor from a wrecked Volvo in the scrapyard behind the bush: Maybe i could replace the cylinder head with a flat steel plate and weld some piping on it with rubber balls that act as inlet and exhaust valves: Then drive it on PTO and i have a super duty water cooled air compressor...

Well i dont have that much time to spend on projects like that.


The 6 CFM, is that 6 cfm of suction air, or 6 cfm at 6 bar ? the manufacturers dont allways state the pressure with their cfm rating...
6 cfm at 6 bar would be roughly 1 cfm at 6 bar.....

 

[sandman2234]

Sandblasting is not for people with small compressors or limited time. A sandblaster large enough to do a large job rapidly takes a huge air compressor, but one like that can be rented for occasional jobs. Check into renting the compressor and see if it is still afordable, then find someone who pays unskilled help minimum wage and let them do it.
David from jax

 

[Bird]

The 6 CFM, is that 6 cfm of suction air, or 6 cfm at 6 bar ? the manufacturers dont allways state the pressure with their cfm rating...
6 cfm at 6 bar would be roughly 1 cfm at 6 bar

Most of the compressors, either on the compressor itself or in the manual, list a CFM or SCFM rating at 90 psi and at 40 psi.

 

[SPYDERLK]

I was just thinking: i have an old 1.7 liter gas motor from a wrecked Volvo in the scrapyard behind the bush: Maybe i could replace the cylinder head with a flat steel plate and weld some piping on it with rubber balls that act as inlet and exhaust valves: Then drive it on PTO and i have a super duty water cooled air compressor...

Well i dont have that much time to spend on projects like that.


The 6 CFM, is that 6 cfm of suction air, or 6 cfm at 6 bar ? the manufacturers dont allways state the pressure with their cfm rating...
6 cfm at 6 bar would be roughly 1 cfm at 6 bar.....

It is 6cfm of suction air. The atmospheric pressure is the Standard in SCFM. The reason this goes down with rising pressure is because of the limited compression ratio of the compressor. The little portion of air left at the top of the piston stroke must expand back to atmospheric before the compressor can start to breathe in. This point comes later and later as pressure builds. You will notice that this shows prominently on single stage compressors and much less so on two stage type. Multi stage compression yeilds an extremely high effective compression ratio and thus will not show this dropoff much at normal 150 or 160PSI pressures for compressed air. My Kellog American 2 stage is rated to 250 PSI. Up there it shows the dropoff - but that pressure is dangerous for everday use. I just crank up the pressure when I need it.
You will need a compressor capable of 20 SCFM delivery at 100psi for extended sandblasting. That is a 5 to 7HP 2 stage. And true motor HP -not that special duty overrating trick they use on the cheaper little units sold for the hobbiest.
Larry

 

[Bird]

not that special duty overrating trick

I never quite figured all that business out. At one time, I had a 3.5 hp Craftsman compressor and the next year, the identical unit was rated (or advertised) as 4 hp. Down on the farm, I had a Puma air compressor, 6 hp, 60 gallon tank, v-twin compressor. But the salesman at the place I bought it from said they referred to them as an "imitation 6 hp" and he showed me one of their "true 5 hp" compessors with a motor at least twice as big as the so called 6 hp I bought. Then later, there was a lawsuit and the compressor manufacturers changed their ratings. What appears to be the same as the 3.5 or 4 hp compressor I once had is now called a 2 hp, and the compressor I'm currently using that is much more compressor is called a 1.8 hp.

 

[SPYDERLK]

That motor rating trick is related to the special duty involved. I havnt figured it out either and Im not gonna try. Im sure it makes perfect sense when explained, however does not pass a discerning reality check. I just multiply Volts X Current and divide by 1000. That, allowing for 74.6% efficiency, is the safe output HP for a motor with a Service Factor of 1.0.
If it comes out much higher than that they are totally playing games unless the SF is greater than 1.0. [SF X Normal nameplate HP gives the max hp the motor is capable of producing safely] In this case their trick would be doing the multiplication 1st and putting it as the nameplate HP.
Larry