On Mon, 28 Nov 1994, Dennis wrote:
> I think I'm going to get a compressor in the very near future. Sears has
> one on sale: 4.5hp 25 gallon (I think) and its $299. It runs off 120V but
> can also run off 240V.
>
> My wifes father told me that this size compressor would not produce very much
> sandblasting pressure. he said he used (15 year ago) to have a gas powered
> one and it would hardly blast enough to strip the rust and paint on a
> horse trailer. The Sears guy said it would sand blast fine.
>
This subject keeps coming up; I wish someone would research a definitive
answer for a FAQ entry. Maybe I will, if nobody else will...someday.
The answer is, it all depends. Horsepower is not the only issue; the type
of compressor and many other variables enter in. Not all horsepowers are
created equal, in terms of air delivery. Over the years, the advertised
horsepower of Sears compressors keeps going up and up. I suspect
horsepower is cheap, and not altogether real, while efficiency is
expensive. Anyway, horsepower is the input, and you should be concerned
with the output: how many cubic feet per minute (cfm) the compressor
delivers at the pressure needed for the use in question.
The air requirement also depends to a great extent on the efficiency of
the sandblaster. Siphon type sandblasters are cheap, but do a relatively
poor job for the air they use. Pressurized sand blasters (in which air
pressure is applied to the sand reservoir in addition to being used to
carry sand from the nozzle) are more efficient in their use of sand and
air. The air requirement depends on the nozzle size, as well as on the
design of the blaster; big nozzle = big air requirement (and fast work,
and more tendency to damage metal).
In automotive sandblasting, you do not necessarily want a huge, ultra high
pressure, high delivery sandblaster like those used to etch tombstones.
One of these might be fine for a horse trailer, which typically has 1/16
or thicker sheet steel to withstand horse pee, but it would destroy 20
gauge automotive steel in a trice.
For automotive sheet metal, I'd recommend a pressurized blaster with a
medium to small nozzle, operating at 90 to 100 psi. You will find units
advertised in Hemmings. Pick a couple of representative units and call
the companies to find the recommended cfm @ 90 psi with an intermediate
size nozzle. Compressors usually state the cfm @ 40 psi (typical for
spraying) and at 90 psi (sandblasting, air tools). Compare the
sandblaster requirements with the compressor output. What I think you
will find is that more or less continuous blasting will require at least a
compressor sold as a 5 or 6 HP 220 v unit.
But you may be able and willing to put up with something less than
continuous use. I suspect that, if you want to sandblast small parts, or
little rusty patches on a body, the Sears unit will work well enough.
Just realize that, if the blaster requires 15 cfm and the compressor
supplies 7.5 cfm, you are going to spend half your time waiting for air to
be stored in the tank. Also, you are going to start blasting at something
like 100 psi, and you will probably keep going as long as the rust is
disappearing. When pressure has fallen to about 40 psi, you will notice
that sand is coming out, but rust isn't going away, and you will stop. A
compressor that would maintain a steady 90 psi, keeping up with the
blaster, would use less sand and a lot less time.
That said, I did a great deal of sandblasting with a 220 Volt nominally
2HP Sears unit. It was a deluxe compressor that they sold a few years
ago, and I suspect its cfm ratings are probably as good as the 4.5 hp
units they are flogging now. Mine, for example, requires 220v; it cannot
run on 110v. Anyway, it worked. I had to spend a lot of time waiting for
it to bring the pressure back up in the tank, but the job did get done.
Of course, as I waited for pressure to return (hot and sweaty and
exceedingly dirty and breathing through a rubber mask) the foremost
thought in my mind was, "I need a bigger damned air compressor."
The last thing that should be said is that having too small a compressor
will limit the amount of sandblasting you want to do, which is not
necessarily bad. Sandblasting should not, IMNSHO, be used to remove paint
from acres of sheet metal, unless you like wavy sheetmetal. NEVER
sandblast aluminum panels like the bonnets of MGAs. Use sandblasters to
remove rust, when you must, to clean complex shapes that are difficult to
clean otherwise, or on frames or other areas where the steel is heavy or a
little distortion won't hurt. Use less destructive methods to remove
paint from low-crowned or flat exterior surfaces.
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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