Did you use any chemicals when you washed? Most pellet stove soot I come across comes off with my basic housewash mix (bleach and elemonator)
If you’ve already tried some housewash mix, I would try using a degreaser like F-13 gutter grenade from pressuretek.com. Follow the dilution guidelines. Work from the bottom up, and keep the surroundings wet. It will be time consuming, but not nearly as bad as a magic eraser.
If the F-13 doesn’t provide satisfactory results, then replacement is probably your only option.
