For years, service technicians and homeowners have acid washed D.E. grids. However, cartridge elements do not have the protective D.E. powder coating that grids have and can be permanently damaged by the acid. As a rule of thumb, acid wash only when required, if at all, and never without thoroughly cleaning the element with a cartridge degreaser first.
After thoroughly cleaning the cartridge with a degreasing agent, flush the cartridge with a garden hose to remove all traces of soap, etc. Now apply several drops of acid directly to a small area of the filter media. If bubbling occurs, this indicates a build-up of calcium or minerals, and you should proceed with a mild acid soak (see Unicel Cleaning Instructions for more details). If no bubbling is observed, your cartridge most likely does not require acid washing.