I believe advice is to stick the drum in a bucket of hot water for 30 minutes before spraying - should make it flow / spray better.
I assume you don't mean cover them with waxoyl! The most important area is INSIDE the chassis and bulkhead.
I would imagine that enough is enough and more is no better, but not enough is worse (if you follow me). So err on the side of too much, but don't get silly. I would imagine getting it as clean as possible is a good start, so perhaps pressure wash on Saturday and waxoyl on Sunday. That's what I did with the Disco, except I got distracted on the Sunday and therefore only actually washed it...
Tim Hobbs
'58 Series 2 '77 101FC Ambulance '95 Discovery V8i