I had lost hope with my electric cooking plates. The white circles where completely hidden under a layer of diamond-grade burn residue that no amount of scrubbing with chemicals could even begin to remove. I found this 3€ scrapping tool and it’s amazing !!! Sorry, but I don’t have the before picture, believe me after 6 years of usage, it was bad.
Oh, I tried that. It had negligible effects. I tried a bunch of different solutions, combination of chemicals, let soak overnight, etc.