Wash with warm water and a heavy duty cleaner. Use a brush to get into the grooves. Rinse, dry. Use car wax. This helps repel water and protect the paint.
a power washer work well for rough cast iron type things...just be sure the paint is intact and you do not get too close.
Ours is that wire mesh type that is impossible to just wipe clean so we took it out in the front yard where we get plenty of sun....washed it with warm water with dishwashing soap using a soft scrub brush. Then we moved it to a dry spot and let it sun dry. I wound up painting mine all different colors. If yours is old, be sure and check for rust spots and do some preventtive maintenance.