Some gunk can't be easily wiped away in the microwave, especially if it's been cooked into a hard chunk of sticky, old food. If you can soften it up a bit first, though, an old credit card can help you finally chip it away. This video from the Consumer Reports YouTube channel demonstrates some microwave cleaning tips we've talked about before, like using lemons to freshen up the inside, but it also has a tip you might find useful if your microwave is really dirty. You can use an old credit card to scrape off the really stubborn gunk that won't wipe away. So microwave some lemons, and maybe some bicarbonate of soda, in a bowl of water, then wipe it down. If anything is left, bust out a credit card and use the edge to carefully peel it off.
The Best Way to Clean a Microwave [YouTube]