From: John De Armond
X-Source: The Hotrod Mailing list
Date: Apr 1992
Subject: Re: rust stripping

>I don't sand blast very many body pieces because it can warp thin
>sheetmetal. It also gets into everything, down in cracks and seams
>that can cause moisture to get trapped and cause more problems. I've
>heard of some places using walnut shells or plastic pellets to sand
>blast with that doesn't warp thin metal like sand does.


Seems to me like the main reason blasting warps metal is that it
locally heats the impact zone.  I have an attachment to my
high pressure washer that sand blasts using water instead of air as
the propellent.  Very effective and no heat.  I've not experienced
any warpage using this device. Available from Northern Hydraulic for
about $50.

John

From: John De Armond
X-Source: The Hotrod Mailing list
Date: Apr 1992
Subject: Re: sand blasting

>I thought the reason why sandblasting warps metal is because it slightly
>dents the surface which pushes metal out around the impact area. The metal
>tries to recover and responds by shrinking. If only the surface of one side
>shrinks then the result is a curved or warped piece of metal.  I doubt that
>one could identically match both sides and come out even, however.

I'm sure that if you lam down 90 degrees on a piece of metal with 200 psi sand,
denting can be part or most of the problem.

>I know that shot peening is somewhat like this, so actually I might have it
>backwards. Doesn't shot peening yield a compressed surface? Then when the
>part is loaded under tension, it can take the normal tensile loads plus
>the extra amount the peening provides. Of course the peening also prevents
>cracks from growing due to the irregular surface.

Yep.

>So maybe sandblasting would cause the surface to grow...
>either way I don't think its heat that causes the problem.


"Sheet metal Handbook", Ron Fourier, HP books covers this topic.  It is
also quite easy to test on a scrap piece of metal. Put your finger behind
the sheet and blast away.  Note how hot the metal gets.  Or if you're
using a bead blast booth, blast a finger of the glove and note how
hot it gets.

Supporting evidence is the fact that water-based blasting deliveres
much more mass to the surface (1500 psi, 3 gpm in my case) than air
and yet it does not distort the metal.  Unless one blasts a fragile
part that is :-)

John

From: John De Armond
X-Source: The Hotrod Mailing list
Date: Apr 1992
Subject: Re: rust stripping

>when water is used for blasting do you still have a problem of it
>(the sand) getting trapped in cracks?

Yes but not as bad.  I don't generally blast large areas that have joints.
I use it mainly for removing rust spots, preping rusted through
areas for welding or lead wiping and so on.

>do you have a number for northern hydraulic?

The local number is 404 590-1114.  They also have a nationwide 800 number
that I don't have handy.

John