Magnetron has 2 small holes in antenna due to previous wave guide being dirty.Is the microwave still safe to use,no sparks or arcing.

Determine the precise location of the arcing. Perhaps the container with butter had metal in/on it. Does this occur with a cup of water?

It is likely at or near the mica cover over the microwave magnetron antenna. An accumulation of soap, grease, oil and food around or behind this shield will eventually cook down, and burn, as it converts to carbon. The element carbon is metallic and microwaves do not get along well with metals. Due to the close proximity of this shield to the magnetron antenna (usually less than 1/2 inch) the power is intense and arcing occurs. In a worst case scenario the metal band at the tip of the mag tube will melt, requiring the magnetron tube to be replaced.

Gently remove the mica shield, inspect, clean any debris, re-coat burnt or bare metal with appliance touch up paint, allow to dry and reinstall the shield, While the shield is off also look at the mag antenna to spot any burnt of melted issues. A bright light and a small mirror are useful for this task.

Mica is a delicate material and must be handled carefully - or replaced.

Hello Stephanie.
Get a new filter and try again.
Otherwise scrap the micro.
It the filter plate rusted through, or is it burnt?
The microwave's Magnetron-wave-guide is just behind this filter plate.
Looks like a small tube with a silvery cap with hole in center.
Very hazardous condition here.
The high energy from it is damped by the filter.
Is there food in the micro when arcing happens?
An empty micro will arc.

sometimes a bad relay on the main timer board will tell the unit to heat with nothing in it and it will arc in the wave guide or antenna area. Arcing sounds like growling or grumbling. "Random" may be from the relay sticking sometimes and then sometimes not. Will destroy the wave guide and the magnetron and the unit could not be repaired. Would have to be replaced if that happens

replace the de-fuser panel with a new one
and try again
if your getting sparking around the de-fuser panel or on it
then its seen better days

you can also clean it with IPA {iso propile alcohol or another solvent {not water !!!!}
then disperses fast and totally
to wash off any surface contamination causing arcs

The antenna on the magnetron is wearing out becuase it is damaged. Replacing the antenna from another magnetron normally solves his problem.

Q - When there is small amounts of food in the oven, arcing occurs right behind the plate where the wave guide comes into the oven. Sometimes it works well and sometimes you can't even put in a bag of popcorn. Any ideas? Could food have 'dripped' in here and be causing a microwave short?
 
A - Yes! Food splatter or drips can continue to cook and will carbon over creating sparks/arcs from the wave guide cover. Usually can clean up the food on and under the wave guide cover, if the oven liner has been damaged we usually lightly sand and touch up those marks with touch paint. Do not let it go for too long as damage to the magnetron end is possible as well :(   If the wave guide cover has been damaged, replace it!

1) How is the cavity damaged? Is it just peeling paint, or is it rust? If it's rust then the microwave could be dangerous.

2) The antenna cap usually has a small hole in the centre, but if there are any other holes in it the magnetron itself could be damaged, and holes caused by the magnetrin arcing.

it is not safe to use this, DO NOT USE, repalace immediately, no need to repaint

If it is burning and arcing at the antenna, it is probably arcing in the wave guide area. Probably will have to replace unit. I have had some success by removing the antenna, sanding and repainting the burnt areas of the cavity and sanding off any burnt marks on the antenna. Need to look at top of mag to see if also arcing there too.

If you're hearing a crackling noise and don't see any sparks, you may have a magnetron that is arcing inside the waveguide. Once this arcing occurs, in some cases, replacing the magnetron may not be enough to fix the problem. There was also a noted problem with some models where the stirrer motor shaft inside the waveguide would arc when the magnetron was energized. This too, caused damage inside the waveguide. If the internal walls of the waveguide become damaged it cannot be repaired and the unit must be scrapped. The waveguide has a special coating to help propagate waves of RF energy. If the walls are damaged due to arcing this special coating gets burned off and will continue to arc even if a new magnetron is installed. This will eventually lead to magnetron failure and potentially cause a fire inside the oven. I would think seriously about looking into another unit if any of this may be occurring in your oven. Better to play it safe, especially with small children. I hope you find this advice helpful. PS A new magnetron can run as high as $250.