Waveguide Cover Microwave Waveguide Cover?

Microwave waveguide cover? - waveguide cover

We fed Frigidaire microwave (sparks) and one night I had sausages over the fire. It was great, but left a hole the size of an eraser on the cover of the waveguide.

I ordered a replacement cap and waveguides by searching online that you can sand the burn marks of the cavity and the use of special paints found to cover them. There are only two brands and small.

For those who have knowledge in this area is to be treated safely and to continue to use the microwave? I fear that the use of sandpaper to create small chips of metal. Someone told me that if the layer of the waveguide must be replaced, the microwave is probably broken anyway. However, we are still good in a different form, and only two years.