Jake, where do you call home? I know guys all over the place that have them for sale on occasion, but may be better to steer you towards someone local that you could try something out before purchase, especially a vintage PH.
First, a cautionary tale, A friend of mine had some doors to repair as part of an overall paint job on a Model A. He had bought a reproduction of a Milwaukee planishing hammer, and after cutting the brace out of one of the doors, planished all of the dings and dents out to a more consistent finish. The other door was too far gone, and he brought both to the shop so we could make one door skin. When I looked at his planishing efforts, although straighter than it was before, the planishing also stretched the piss out of the entire door skin that it bulged outward, with a noticeable step starting exactly where the planishing stopped along the edges. So we ended up making two door skins so he would have less of a mud job:
Scott from Kahiko Customs in Charles City VA showed up early yesterday with two doors for a Model A. 90 years or so of carnage has taken it's toll and we were collaborating with him to make some new door skins. Given that we opted to go ahead and make both door skins to have a more...
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Point of all this rambling is that he made a poor choice in die selection, one with too much crown and it added more stretch than needed, making the panel unusable. A tool like this is primarily used for planishing long runs of welds where repairs have been made, and to a lesser extent, to make things flat again. In much the same way as one selects a dolly for bumping dents, the one he should have used would be as close to the crown of the existing panel as you can get without the edges of the die touching (to prevent "coining") For straightening, I would also suggest using about the lowest pressure the tool will stand, again, to limit any stretching. (think of air pressure here the same as exertion of a hammer swing) So where a long weld such as filling a roof insert on a 34 Ford, the Planishing hammer works extremely well to stretch out what has shrunk from welding, bringing it back to the original crown. For something that is already damaged/dented, 99% of those will already be stretched, so care needs to be taken to prevent adding MORE stretch in the panel. Using a tool like this, things can get out of hand quickly.
For a BRAND NEW reproduction of the infamous Milwaukee planishing hammer, Jason Millard (JRM Metalworks, JRM Tool CO.) has quite a few pictures on Instagram of the ones they reproduce:
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Fully outfitted with a good selection of dies and hoops you can be a few thousand or better into these tools.. So yeah, if looking at used, I would want to kick the tires before exchanging money..
So most of the louver jobs I have done have been in collaboration with Cody Walls of Traditional Metalcraft in Milton DE. He has a Clay Cook Pro-Line planishing hammer. Cody says he uses it on an almost daily basis, for the past 13 years or so. Hasn't skipped a beat, he's very happy with it:
The ProLine Portable Pneumatic Planishing Hammer is designed for forming or planishing compound curves in sheet metal. These hammers are completely fabricated in the USA.
www.trick-tools.com
If you are on facebook, there is a planishing hammer group, they have some come up for sale on occasion. It's a closed group, so let me know and I can put you in for membership.