I did my last chassis with dinitrol, there ain't nothing getting behind that, I couldn't believe how well it stuck, much better than waxoil
Luckily we had a period when it didn't rain for a week so I knew it was dry when I did it but still gave the nooks and crannies a squirt with an airline first
Another vote for dinitrol for me, great stuff. If you decide to go down this route I made a thread on what I did somewhere
On the important issue of washing your bottom, I did see a farmer who had installed a set of water jets in a corner of his yard. Drove his 'fender over them, turned on a tap (timed shut off, I think) went and had a coffee and the job was done.
Used it everytime he went in a field or on gritted road - so easy that it always got done.