appliance stores sell a special paint, probably mostly special so the
material doesnt outgass when heated and contaminate your food...

might be time to consider a new microwave, yours is getting elderly..

at least run your finger around the closed door while the ovens heating
, if you find your finger gets warm get oven serviced or replaced.

Old appliances can become a hazard, espically microwaves...

hinges wear and seals go bad after nearly 20 years.

BTW I tend to hang onto stuff forever, but old semi hazardous ones
concern me