Re: Digest Number 2013


Malcolm H. Houck
 

In a message dated 8/21/2004 11:10:25 AM Eastern Standard Time,
STMFC@... writes:
Which ever you use in grit-blasting cars, it is best to wear a mask to
prevent inhalation of whatever media you are using. A little precaution
goes a long way.
With a blasting cabinet the media material is contained, falls to the bottom
to be recycled through the blast nozzle. That purpose of the "cabinet" is
avoid the need for respiration equipment in the first place. Some media leaks out
[via the pressure release vent], so it's not a bad idea to wear a paper mask,
but not much more is needed if the cabinet is used with care.

So far as media is concerned, sand "wears out" by breaking up as it's used.
The fastest "cutting" is with the maximum surface area in the media. Play Sand,
being rather coarse, in addition to the disagreeable feature of the particles
actually shattering [which is also a feature of beads; -- they break up too,
and "wear out"], just doesn't cut as fast as a finer material. That notion may
seem contra-indicated, that finer material cuts faster than material more
coarse, but that's my experience with blasting media.

Baking soda is the preferred media used by commercial firms whose business it
is to strip fiberglass (Corvette) automobile bodies. It's extremely fine, and
tends to be messy, from what I've observed of its use commercially.

Blasting in the cabinet successfully is a combination of both media and
properly regulated air pressure. Yes, indeed, brass models {especially the
"imports" that use a very soft and limber brass} will warp if approached without care.
I've found that warping is usually caused by both air pressure that's too
high, and media that's too rough. Sand is a special culprit, as a media, in brass
models warping.

I like to use #340 or #440 grit ALOX [Aluminum Oxide]. For brass an easy 30
psi cuts with no damage at all, albeit at a slower rate that with higher
pressure. Paint removal is easily done, but will slow dramatically if the paint is
"soft." Blasting Floquil paints that aren't years old and well cured will be
slow, at best, and will tempt one to turn up the pressure. Scalecoat comes off
quite well since it cures quite hard. On brass models I'll give them an
overnight soak in MEK, and what doesn't dissolve or float away is no longer securely
bonded to the parent metal and blasts away very rapidly.

On plastic there's little trouble with 30 psi, or even less, for paint
removal or simple preparation. I keep the material nozzle at about a forty-five
degree angle to the work and have little or not problem with complete cleaning.
The 440 media will put some texture to the plastic, but that will only make
removing the new coat that much more difficult due to an enhanced mechanical bond.

I like to machine the blast nozzles of the gun 1/4" in diameter from free
machining 2101 steel. I heat it cherry red, after turning and boring, and then
oil quench it to dead hard. I don't bother to temper, though it can be easily
done at home in an oven or toaster oven. 400 degrees F will temper 2101 steel to
C62, about the hardness and toughness of a woodworker's chisel. Put a pinch
of table salt on the tray with the item being tempered and when the salt melts,
it's 400 F. But, as I stated, I've come to believe that's not worth the
trouble to temper these little nozzles. The abrasive media will wear through the
nozzle in not very much time, and that will dramatically slow the cutting of the
media in the blast cabinet, so there will be a need to either purchase or
make new replacement nozzles with some regularity.

All of this is likely much more than any one wanted to know, but it's based
on my experience with the use of a blasting cabinet for the last thirty years.
If anybody needs to know more, contact me off the list.

Mal Houck

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