Interior damage to a GE microwave that affects performance is impossible to repair. Minor chips in the interior coating should not influence the performance of the unit. However, if the coating is peeling, it's time to stop using the microwave.
If a hole forms in the cavity wall, it is time to replace the product. This is one possible effect of arcing inside the oven as a result of metal touching the side or incorrect placement of the metal rack on its supports. While the cavity coating is safe as a surface for food contact, once it starts to come off, replacement is a must.