Hi Lee,
The spray coat of paint on the oven cavity is not important to the safe use of the device. However you dont want flakes of paint in your food, or rust patches forming.
The simple solution for most microwaves is this, clean the affected area with a damp soapy cloth, dry it well, wipe the area with methylated spirits or IPA on a cloth. This will dry the surface and remove any last grease spots. Leave to stand for 10 minutes to allow any residual spirit to evaporate.
Obtain a small pot of Humrol type model enamel paint, like you use for plastic kits, apply very thinly with a small modelers brush, leave to dry naturally, repeat enough times to leave a satisfactory new "patch."
To prevent this condition in future, wipe the bottom of the cavity with a damp soapy cloth every time you use the oven, wash both the glass turntable (underneath especially) and the roller. The explanation for this is too long.
Contact me via microwave-repairs.co.uk if you need further help.
Patrick, October 2010
