Rashers wrote:

What is the advice for a Dinitrol application if the vehicle has been previously treated with Waxoyl? I've often wondered. Mine was treated before I got it (by the dealer). It still looks pretty good underneath, although the trip across the oil seed rape stubble last week did mark some of the underside. Would it just be better to stick with Waxoyl?

Same applies light coating of cavity wax then underbody wax. Make sure the cavity wax has gone off first however.
You are basically spraying on epoxy (glue) which seals and sticks to the surface you are spraying it onto and the underbody wax which is also epoxy based sticks to that although the latter is thicker/waxier, less runny and goes off harder