POR-Strip on underseal?

I think you've just reminded me of the new seam of rust I struck when last working on my Triumph (sometime last autumn..). Was cleaning off some dried out seam sealer with a power scraper, and.....

Assuming it's the same type of thing (buzzy hand held thing, with chisel like "blades" which vibrate and thus shift whatever you need shifting), it did very well aside from being somewhat assertive with what I had taken to be not rusted through metal, and also sometimes getting stuck on lips and rough bits and trying to chisel its way through.

Not tried the POR stuff, but a blowlamp, scraper, and then celly thinners and wire wool went fairly well at getting underseal off an inner arch. Took at least a couple of hours though.