Good old WD-40...
I read something, so I tried it, and it worked wonderfully. Half the time, it's just about cleaning the glass doors. I sprayed WD-40 all over the inside of the glass and wiped it with a fibre cloth. It took a bit, but it cleaned all the hard water, soap, and scum off the glass instantly. Now, I do this weekly, and the glass is getting easier and easier to clean.
If your grandkids write on your walls with crayon. marker, or pencil... Spray WD-40... and it comes off. It will also remove paint from metal.
Also, it is great for taking the bugs off the front end of our vehicles and will not hurt the paint...I've tried all that I wrote, and it works perfectly...
WD-40 is only flammable in the can... Will not burn once it is sprayed on wood, metal, or paper...
Cheers~