The answer given by 'the winner' is wholly correct. There is no remedy, but drying with kitchen roll definitely helps. Trapped steam - not burnt food - lifts the paint and corrosion sets in. All microwave ovens with a painted interior surface will eventually fail like this. I used to do bulk catering. I have gone through at least ten ovens that have failed in this manner. What eventually happens is the mild steel beneath the paint rusts. This rust then start to get tiny electrical arcs from the magnetron which then gets worse over a quite short time. This visible sparking then will pierce the inside which then makes the oven unusable/dangerous.
Solution is keep buying new ovens.
Buy a quality oven with a stainless steel liner as suggested. Cheap stainless steel will also fail.
Or experiment with expensive high-temperature paint aerosols.
I used to buy microwave ovens from Sainsbury. Use them for ten months and return them as soon as the paint starts to bubble. They once sent an engineer, who took one look and sent a brand new one. He said that it wasn't safe, so took it with him and a new oven arrived the next day.