So the preference by other builders is to use Dinitrol 3125 and spray the engine compartment, grot slots, behind the fuel tank and inside the de-dion. Then spray under the floor with Dinitrol 4941. Finally make two infil pieces to stop rubbish from entering the grot traps.
Other useful tips seem to be to spray inside the chasis if you happen to drill out a rivet. Also some people have talked about sealing between the body panels and the chasis tubing with sealant.
Another common tip seems to be to paint the underside of the wheel arches to prevent star marks in the paint from stones.
Finally polish everything before you add any bits to the cars.
After all that, start building.