Depends what you are doing with the metal afterwards. Are you recycling the sheet metal or is the plan to remove the foam and replace with new foam?

But, I have seen people scrape it off with different types of tools. I have also seen it done by sandblasting. Depending on how much of a area you are talking and what is being done afterwards.

The sandblasting maybe the way to go.

What ever you do don’t burn it. If it is Urethane foam is a thermoset. Thermosets cannot be heated and changed back into their original liquid form. When urethane burns, it does not liquefy and run as many other plastics (thermal plastics) do. The urethane either burns very hot (such as an exposed building lining) or it doesn't burn well at all. There is no danger of urethane foam smoldering. The real fire danger from urethane comes from its insulation value, since it increases the temperature within a fire area.

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Do you know if the foam is held in place by studs or is it simply adhered to the surface? I you've got studs (they will probably be no bigger than a small wire) your best bet is to go at it with a large hand scraper. If no studs use a hoe or roofing scraper.
Also how thick is the sheet metal? Is it laying flat or is it vertical? If it is laying flat and reasonably robust, you might be able to use a roofing cutter. We use these to remove built up roofing for abatement or prior to demo to reduce fly debris. It's kind of like a mower with a verticl blade. You would still have to scrape it. A skid steer loader with a straight bucket would work for large flat areas. Vertically you could use a loader bucket (no teeth).
Also remember that the foam will need to be disposed of properly, especially if it is hazardous. It's always a good idea to get it tested for asbestos.

If the foam is hard it's easy to do with a demolition/sorting grapple on a excavator.

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That works also!

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Pogo stick or air spud?