Hi, my name is ***** ***** I have been in the home improvement and repair field for 20 years. I will be very glad to help you today. If the floor is cracked through then it could lead to further low radiation leakage and also possible fire if not properly repairing or replacing. If only the surface is damaged or rusted then unplug the unit and wipe the damaged surface with a soft rag to remove loose flakes of rust.If the rust is under a glass shelf, remove be shelf. If the shelf is held by sealer, use a razor blade or utility knife to slice through the sealer. After the shelf has been removed, clean off any strips of sealer that remain.Buy a spray can of enamel paint for your make of microwave oven from a dealer who sells your type of oven. Also buy a tube of RTV [room temperature vulcanizing) sealer if the unit has a glass shelf.Following directions on the can of paint, refinish the damage. Let the paint dry.Install the glass shelf. Then, following directions on the tube of sealer, apply a bead around the edges of the shelf.Before using the oven, let the sealer cure for the length of time specified in the instructions on the sealer tube.