As you discovered, hammering stretches the metal. Unless you hit both
sides equally, it will warp. BTW, this is how they used to make boiler
tanks in dirt-floor countries. They started with a flat plate of steel
laying on the ground and a big hammer. A guy stood in the middle of the
plate and whacked away, and eventually the sheet was curved enough to
make the ends meet.

ben carter wrote:

Well, I tried a pistol grip impact hammer with a bit ground down to a
good radius. My compressor was a bit underpowered so the depth and width
of the indentations was inconsistent. If the difference were random it
would actually look good but there tends to be lots of ugly grouping of
patterns as the compressor tried to keep up. After half an hour the
compressor seized up anyway... The 12" x 12" x .125" plate of 5052 was
also warping as much as .5" over 4" of the pattern width. Not so good
considering the final piece will be 42" x 106". I'd have a cylinder!
So I tried a needle scaler. A Chicago Pneumatic cheapie with .125"
needles. The difference was dramatic in both consistence in depth of
dimple and amount of surface I could do quickly. The distortion was way
less too, only .25" over 12" of width.
Any suggestions on how to flatten out the sheets after I texture them?
That's still 10.5" over just the width, 26.5" over the length. I tried
reverse rolling the 12" samples through a hand rolling machine without
much luck.
It seems that the 5052 might be dulling the needles as well. Is it worth
trying to resharpen them? Perhaps by grinding them flat again or something?

I'm still open to ANY reasonable suggestion to do this differently.

Sorry for being long-winded, this is really important.

thanks all

Ben