People seem to have many different ideas regarding "outside" care for a healthy tortoise, that is, the part that he lives in!
One thought is that since turtles are "wild" animals, they needn't be bathed. Exactly the opposite is true.
Another misconception is preparations of some type (many of which are commercial) are recommended, to be placed on the shell, to "give it nutrients" and make it shiny. Many, if not most of these products, are potentially harmful.
Both the carapace and the plastron of a tortoise or turtle is comprised of living tissue. It needs oxygen and exposure to sunlight to keep healthy.
The build-up of soils, waxes, oils, or other preparations, is actually harmful to a tortoise's shell. The keratin and bony layers contain millions of microscopic pores, and things such as the products mentioned actually clog the pores, causing problems.
It is perfectly safe to give your chelonian friend a bath, in fact he/she might actually like it!
All one needs is a toothbrush, a tub of some sort large enough to accommodate the tortoises, and tepid water, no deeper than is necessary to cover the entire plastron, and a few centimetres of the carapace.
Allow your tortoise awhile to rehydrate itself, usually 15 to 20 minutes, then replace the water. Often, tortoises will often take advantage of this time to replace their water stores, voiding the old, and replacing it with fresh.
Take the toothbrush and gently brush the entire carapace, being particularly attentive to any areas of build up, between the scutes, and beneath the carapace as well, to include all areas above the head, surrounding the legs, and around the tail.
Repeat the same for legs, tail, and neck. Be careful of any areas not covered with scales, as these areas can be more sensitive.
Repeat the same procedure with the plastron, paying careful attention that all dirt is removed from between the scutes.
Finally, rinse well. DO NOT use any type of detergent, shampoo, or other product unless recommended by your veterinarian. Some of these products, while safe for dogs, cats, or people, can be potentially harmful, and sometimes even fatal to turtles.
Turtles and terrapins, even though most always in water, appreciate a shell cleaning on occasion as well. Algae, shedding skin, and normal peeling of the scutes in paper-thin fragments can all be removed from the turtle, allowing for healthier shell growth.
After your turtle is clean and dry, if healthy and on a good diet, should have a shell which shines all by itself. No additional treatment should be necessary.
This procedure gives the owner an excellent opportunity to examine the animal in detail for any problems, such as injuries, shell rot, or abrasions.
Regular, monthly cleaning and inspection make for a healthier, happier turtle, with less chance of potential for skin or shell problems.