My microwave oven is rusty underneath turnable so is it safe to paint it &which paint to use

see the troubleshooting section and follow the steps and fix it.
God bless you

Waveguide Cover

If the microwave is sparking or arcing the waveguide cover might be damaged and shorting out. Try removing the waveguide cover and see if the problem goes away. If there are any damaged spots in the microwave ceiling or sidewalls, repair them and paint them with microwave oven paint. Replace the waveguide cover if it is burnt or if it will not stay in place.

Microwave Oven Paint

If the microwave is sparking or arcing on the side walls the paint might have chipped causing pitting in the metal oven wall. Use microwave safe paint to touch up any bare metal spots.

Rack Support

If the microwave is sparking or arcing near one of the rack supports, remove the support, repaint any bare metal, install a new support.

try a paint pen used on cars or finger nail enamel :)
1st clean the area sand off rusty bits

Hi,
Here is a tip that will help you to figure out what is wrong with yourMicrowave Oven....

Microwave Oven Basic Troubleshooting Tips

heatman101

You might find your answer here:
http://steprightupmanuals.com/microwave-oven-service-manuals/sharp-microwave-oven-service-manuals/sharp-r1405-r1406-microwave-oven-service-repair-manual

prem

You need to clean it, sand it shiny, then clean it squeaky again.

Then you can get high temperature appliance paint in spray or brush-on form from local appliance part stores or places like Wal-Mart and Lowe's.

I would suggest you let it dry overnight with the door open before using the oven.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

in order to be 100 % sure
you would have to check voltage to the motor while its suppposed to operate
take off the bottom 6 screws, you might need whats called a safe t, a torque wrench or star with a hole
most use phillips,
drop the bottom, you should see the turn table motor
disconnect the two leads and connect them to a voltmeter,,,
turn the unit on
you should read voltage to the meter
if not the problem is in your control board and will need to be replace
if you do read voltage and the motor is not turning
you will need to replace the actual motor
either way its a pretty simple repair and can be done without taking the unit dwon

If the cavity metal of the oven is chipped or charred, it will have to be sanded smooth and clean and dried thoroughly first.

Then you can apply some high-temperature appliance paint.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

Nothing special in there. As long as the paint is not flaking into the food you are ok, Be aware that bare metal could begin to arc. There is special microwave paint available but it will not match and look bad unless you redo the entire interior.
Eric

As long as the waveguide itself isn't damaged, you should be able to replace the shield without issue... That shield only prevents food from splashing onto the waveguide itself and is not part of the microwave RF protection... Check your local hardware store for touch up paint.. shouldn't matter what brand as long as it contains rust inhibitors.. Good luck...

Surely you should have a 12 month warranty with it, take it back to the shop and tell them it's going rusty, and that it's not fit for purpose. Read your consumer right's which are usually available on the internet, then quote it to them if they argue about it. They should either repair or replace the machine.