I agree with Tinus. I think allowing air movement is the best, and a good polish is the best to do.
This is a bit of a messy job, but the guys in Mocambique paint or spray diesel on the undercarriage of the cars there. At Builders Warehouse one can get a 5L pump action spray bottle. fill with diesel and put the car on ramps or on a nice axle twister, climb underneath with a condom over your whole body ( so you don't come out eventually looking like a diesel soaked rag .....haha ), and spray EVERYTHING all the way underneath. It doesn't need to be a heavy coat of spray/diesel, just enough to leave a nice oil coating everywhere without excess dripping. leave it outside for a day to let all the drips fall off and not on your brother in laws shed floor. Alternatively, get the cheap plastic spot sheets from Builders, lay on the shed floor and park the Patrol on top.
When you are back most of the diesel will have dried up. So drive to a carwash place that does chassis wash and Bob's ya cousins dog. Or if the diesel smell is not too bad, leave it as is and drive to your hearts content
Nissan Patrol GL 4.2 Diesel Turbo (Iron Maiden)
Nissan SANI 3.0 V6 4x4 (SOLD)
Isuzu KB 280 DT 2x4 ('ol Smokey) - SOLD
Suzuki TL 1000R "V twin" (Growler) - SOLD