For the benefit of those like me who had never heard of oil-canning before: "Oil canning, a common inherent trait of flat sheet material, is a moderately visible deformation. This is can usually be seen as a ripple or buckle along with the broad flat areas of the material. Oil canning is an aesthetic complaint and does not have any effect on the integrity and strength of the material." [0]

[0] https://stampingsimulation.com/effects-of-oil-canning/