I think I ought to take the route of stone chip coating after finish off using the Jotamastic. I've around three foot from the ground to the underside, I possibly could raise the car another few inches which may allow the usage of the shutz gun. I agree a cavity wax is essential, should this be applied before a stone chip coating is used over the Jotamastic?
as said earlier cavity wax should be the last thing used. if you're spraying it, it will create a fine mist which will escape from the cavities an coat the underside, so you would need to degrease the whole underside before applying stonechip