Re: Any thoughts on sandblasting?

Posted by Carla on July 13, 2004 at 12:41:54 from (165.121.163.34):

In Reply to: Any thoughts on sandblasting? posted by Chris on July 12, 2004 at 16:17:14:

I'll concur with the prevailing opinions.......please don't destroy your tractor by having it sand-blasted.....unless.......you have it completely stripped into bare components, protect any machined surfaces with duct tape, and are willing to do very extensive cleaning to remove all traces of abrasive, including cleaning out all the tapped holes.

(the old books on industrial manufacturing techniques explain the use of huge "washing machines", using boiling-hot water and strong detergents like a household electric dishwasher but of a much larger and more powerful size, to remove all remaining traces of the foundry sand from castings, after the machining is done, before the parts go to the assembly floor.)

I've never restored an old tractor, but I've restored quite a few smaller items of old machinery, and have glass-bead blasted literally thousands of old castings, over the years ...

It is quite easily possible to make cast housings look "factory new" with abrasive blast cleaning, and replication of the original paint finish......but.....sand, or any other abrasive media, will be forced into all the little "nooks and crannies" of the castings....every last trace of this material must be removed, or the oil will pick it up, and carry it directly to the gears and bearings, with the obvious result...

Sand-blasting can work very well on removable sheet-metal parts, or, for that matter, on the generality of cast iron or formed/fabricated steel parts other than housings carrying internal components which run in oil.

That said, there is "no free lunch" with sand-blasting.......blasting thick heavy iron or steel parts is "self-explanatory", right enough, but blasting thin sheet steel components like fenders or engine-shrouding is not easy......the sand-blasting media and technique used for blasting iron bridges and brick buildings will destroy sheet-metal parts beyond repair in a matter of seconds.....

There is an easy way to clean up thin sheet-metal parts without warping or embrittling the metal, and i've done quite a bit of this, one time and another, on such items, as, for example, the shrouding for the little Wisconsin air-cooled engines....first, use a strong paint-remover, and apply plenty of it. I use one or another make of "aircraft epoxy stripper"...its not cheap, but the time saved makes it cost-effective.

After allowing plenty of time for the remover to act, and then dry, blast the parts with medium-grit glass-bead media, taking care to use a relatively low pressure, say 30-ish-40-ish lbs of air at the nozzle, and running the media stream at about 45deg to the work.

The old paint, with its "body" and "adhesion" destroyed by the stripper, will come "flying off", revealing clean steel and rust spots......go over the rust spots til they come clean, taking care to run the abrasive stream from different angles, and avoid steady blasting in any one spot for more than a second or two.

Done carefully, this technique will leave the sheet-metal parts nicely cleaned up with no warpage or embrittlement whatever.

If the sheet-metal had significant rust-pitting, the glass-blast will sometimes not remove quite all the active rust from the pits......go over the work with a phosphate "metal-prep", well "scrubbed-in" with a fine wire brush, to neutralise the remaining rust, clean the metal well with lacquer-thinner, then use an "etching-primer", followed by a "sanding-primer" if desired, before shooting the colour.....for nice work, I'll apply a thin coat of "Devcon Plastic Steel" epoxy putty to any pitted areas, and sand smooth, after the metal-prep etch, but before any primer....

Many delicate small parts, such as "pot-metal" (mostly zinc) carburetter castings, ignition components, or even data-plates, can be cleaned to look "as new" by a gentle blast with "walnut shell" media...."gentle" is the operative term here.....

cheers

Carla

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