If you want to avoid ketchup stains on the plastic, start spraying your clear plastic plates with a non-stick cooking spray like Pam. Do this before pouring into tomato-based sauces. You will notice that there are no more spots or at best a slight coloration. Over time the oil will set in the plastic, but the first few times it certainly looks a little tonality.
The principle behind the plastic layer is that this thin layer of oil will prevent the sauces from sticking to the side of the clear plastic plates in bulk. All you need to do is simply cut a lemon in half and rub the cut part over the spots on the plastic. Make sure that some of the juice is compressed in the stained plastic as well. Then let it dry in the sun for 4-8 hours and then wash with a little detergent to wash dishes. If you see no trace of red try adding a little baking soda for an extra muscle.
If plastic food clear plastic dinner plates are being stained after trying these three cleaning tips, chances are it is time for you to buy a new plastic dishes. Plastic dinnerware and other plastic dishes are cheap and last for years which are an investment that pays back every time you eat a snack or a piece of fruit that is stored in them.