I have rust on the door. The unit is 13 years old. Does it make sense to sand and paint the door, replace the door, or replace the unit. If sand and paint, what type of primer and finish paint. If replace the door how much will the part be and how hard is it to do? If replace the unit, what is the replacement model number

Lots of questions that may be too difficult to help make a decion.

1) Depends on the degree of rust and whether it is inside or outside.
Outside is easier to deal with. Inside it would be subject to heat burning and odour as most are baked enamel finishes and are not paint at all, but a ceramic compund.
Outside can be rust removed which creates its own undercoat as it dries using the rust particles in an altered chemical state. An enamel paint is preferred or even a ceramic based compund.

replacingt the whole door may involve dollars to $100 unless you can find a second hand market for spares.

Frankly, because Microwaves have become so cheap now ( even secoid hand ones) you can pick them up from $50 to 200 depending on size and wattage. i would consider that it has had its time and replace the whole unit. After 13 years of service its time to retire it and move up. Newer ones are more power efficient and better sealed against radiation leaks than the older ones

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