There are a myriad of different shell ointments and creams available. Some are medicated, some are nothing but repackaged vaseline. In the best case, these creams do nothing. In the worst case, they inhibit the natural processes on the shell or make an existing problem worse. My advice: Don’t buy them. Don’t use them.
If the turtle has problems with the shell, such as natural shedding, small spots of shell rot (large spots must be seen by a veterinian, as do sores), or flaking, applying any cream will make the problem worse. Make sure the turtle gets plenty of natural sunlight, basks, and eats a healthy diet. If the problem persists, a trip to the vet might be in order.
Sunlight, rubbing against rocks and wood, and basking (drying out) help keep the shell healthy and help heal small injuries or small patches of fungus. Applying a cream prevents these natural processes.
The turtle does not require shell polish,