While it is technically possible to repaint or recoat the interior of a microwave to address rust issues, it is not recommended, and it can be hazardous. The interior of microwave ovens is specially coated to reflect and contain microwaves effectively. This coating is designed to prevent sparks and other potential safety hazards.
If your microwave is rusting in the cavity where the glass plate sits, it's a sign of significant wear and tear, and attempting to repaint or recoat it may compromise the safety and efficiency of the appliance. Rust can cause arcing, which may lead to damage or even a fire hazard.
The best course of action is to replace the microwave if it is rusting inside. Rust indicates deterioration that can affect the overall performance and safety of the appliance. Continuing to use a rusted microwave can pose risks to your health and safety. Additionally, tampering with the microwave's interior or attempting DIY repairs can void any warranties and may lead to further issues.
To ensure the safety of your household and the proper functioning of your appliances, it's advisable to invest in a new microwave when you notice signs of rust or significant wear in the interior.