I think having it 500mm off the ground will not allow enough work space for you to be able to properly access the underside and remove the stoneguard. The last time I did it manually with a grinder on the guard inners it took a lot of effort to remove the material and achieve the bare metal finish. It will be exceptionally difficult laying on your back working out of place, so difficult that the outcome would most likely be sub standard and potentially a waste of time. If you give yourself the optimum working position you will achieve a very good result.

Sand blasting is the best way to remove the stoneguard and bituminous coating on the underside,