You should hose off your carts when the pressure rises 8 psi or after 1 month, whichever comes first. You can go longer if the pressure has not risem much but the carts will be harder to clean. Use a short, non adjustable, small bore brass garden hose nozzle. It works better than more expensive cleaning wands and only costs about $4 at the harware store. You should shoot the water from top to bottom at a 45 degree angle until the cart is clean.
Once or twice a year you should soak your cart but NOT in acid without degreasing it first, unless you want to ruin it! Use automatic dishwasher detergent powder like Electrosol or Cascade. Use 1 cup for every 5 gallons of water and soak the cart overnight and then hose it off (you need a containier big enough to submerge the cart, ether standing on it's end or lying down). This will remove any organics without damaging the Reemay polyester material the cart is made from. Once the cart has been degreased you can inspect it for calcium scaling or metal staining and, if present, you can now acid wash the cart. (Acid washing will shorten it's life.) to acid wash soak it in one part muriatic acid to 20 parts water until bubbling stops or about 20 minutes and rinse again. ONCE AGAIN, NEVER ACID WASH A CART BEFORE DEGREASING OR YOU WILL HARDEN THE ORGANICS ON THE CART INTO A CEMENT LIKE MATERIAL AND RUIN THE CART!

Hope this is helpful.