I’ve had good luck using simple green to clean up dirty cart shells. Q-tips for the side vents. Yeah it’s a pain in the ass and takes a while but if they’re caked with filth it really makes a difference. If you’ve got some scratches you’d like to buff out try a magic eraser, but note that it will take texture off plastic and make it shiny and smooth.
I cleaned all of my 150+ carts inside and out under the shower:
Took the boards out (without ruining the label) and let them soak. Later cleaned them with a 40 mm wide soft paintbrush and an old toothbrush. Rinsed them and shook as much water off as possible. If it was warm enough, I put them aside and let them air dry. If it was too cold, I used my hair dryer.
After putting the screws back in, I put the shell into one of the smallest IKEA SAMLA containers and let it soak. The water level was high enough to cover all of the vents but obviously didn't reach the label. Used the old toothbrush, the 40 mm wide soft paintbrush and a 12 mm wide paintbrush with stiff bristles to get as much dirt off/out as possible. The 12 mm paintbrush was especially useful for getting dirt out of the corners behind the label on the inside of the shell. (The only risky thing is rinsing the shell without getting water on the label but it can be done, if you lower the water pressure and make sure that the water hits the shell at the right angle. Have lots of paper towel nearby in case you do get water on the label. Using something like a squeeze/squirt bottle instead of the shower head would probably work, too.) Coverd the label with a clean and dry cloth, then shook the shell to get as much water out as possible. Put a stack of dry pieces of toilet paper where I could quickly reach it, then took one sheet, folded it twice and put a small amount of IPA on it. Made sure that it was only slightly damp. Used that to gently wipe down the label and the top of the shell. Repeated if necessary.
Put some of the not so nice looking screws in vinegar for a few minutes before scrubbing them with the old toothbrush.
That process took me about 20 minutes per game and really wasn't much fun but the amount of dirt that came off/out of some of these games was insane. I didn't ruin a single label. No crease marks and no water damage.
Magic eraser will make MVS cart shells (with the exception of Metal Slug 4) look dull. If you feel like you have to use one, then I would recommend polishing the case afterwards. I used Novus 1 2 3 on two or three of my games that had some really nasty scratches. However, polishing the case will make it look shinier than it used to be when it was brand new. That's why I'd only do that with
really bad looking carts.
Wooden (bamboo) toothpicks can also be useful.