Depends on where I was spraying it. Just behind the rear wheel wells, where the shackle box is. I was dealing with a little bit of rust so I used a wire wheel on the rust, then a scotch pad to scuff the remaining paint/primer as well as some JB weld to fill the gaps between the pinch welds, once dry, I scuffed again, wiped with mineral spirits and spray painted. Once the paint was dry ( a couple days later), I scuffed the paint and sprayed thick coats of the undercoating.
On the wheel wells, which I not yet undercoated, I will probably take a dish scrub brush with stiff bristles with me to the DIY carwash, pressure wash the existing undercoating and scrub, wash again to clean off any dirt, dry it over a couple warm days and scuff any exposed paint and then spray the undercoating.
Someone might have advice or insight into a better way but this is how I've approached it. When I say thick coating, I'm spraying this stuff up close and it's almost to the point where it's running. I'm spraying places where I don't care if it has runs in the coating but so far it's been thick enough that it doesn't run while leaving a heavy coat I can dig a fingernail into.
If you're experiencing extremely thin coats, I'd spray closer and be liberal with the stuff. Wheel wells don't need to be perfect.
Hope that helps.