I take back my comment then, since your method apparently works. Has never worked for me, though, doing just as you describe – I could slightly reduce the dent but never wholly restore the profile and in at least one case the oil can dent eventually returned, even though the boat was properly stored, as if it had a memory of the dent. But then I was always working with older boats, 15 to 20 years old, which may have had something to do with the lack of results. Poly gets more brittle with age.
My friend’s kayak is more than just oil-canned, it is warped, literally deformed so that the keel-line is twisted inches off center. Looks like a giant picked it up and squeezed and twisted it!
A search turned up this description of another method, using a heat lamp, oven mitt and ice pack. I expect her hull would have to be heated to just shy of meltdown to be able to reshape such a huge area. Have you ever used a heat lamp for your repairs?