My RF, which I use as my main car all year round, has not had underbody anti-corrosion treatment so in the winter months I rinse the underbody regularly to clear away as much road salt as I can. I’ve seen various postings about using a pressure washer for this but I am finding that a £18 Hozelock lawn-sprinkler does the job very well and is easy to use. See photo.
Through trial & error I’ve found that the half-cone setting is best as the symmetrical ones tend to rinse me as well as the car! Fitted to a hose the sprinkler can be moved around under the car (you have to lie on the ground for this) and it doesn’t tip over when moved. No adaptation from bought condition is needed.
I go in a drive after use to dry the underbody off. If the roads are dry this shouldn’t result in more salt getting onto the underbody.
I use the excellent previous version of the Hoselock lance, with the straight tube and now at least ten years old. It even allows one to replace the worn-out plastic nozzle/clip-on-fall-off-oops bit at the bottom of the handle where the hose attaches. It’s hanging in the middle of my picture below, and will be in use later this afternoon to wash the muddy Mazda3.
Please don’t waste any hard-earned on the current dire offering, the “cost reduced and continually improved” useless, fragile Hozelock Plus 90 lance.
I have a Nilfisk pressure washer and they do an underchassis nozzle that is excellent, although not cheap at £18. It does involve crouching on the floor so you can manoeuvre it into all the places it needs to go, but it does a good job of getting salt off the underchassis and crud out of the wheel arches - to the extent MOT testers have comment on the unusual cleanliness underneath the car
But surely the water also causes rust?
Also what about spraying under the engine? All that water entering the engine-bay does not sound like a good idea to me.
Are the pressure washer attachments the correct pressue because it is said pressure washers should not be used on cars.
Thanks.
What about one of these?
Engines are waterproof
where does it say that I have been using pressure washers on cars for years
Yep, I’d love to see the ‘source’ of that pearl of wisdom… 
I have a wheeled attachment similar to the one @Chris_Ab mentions, as well as an angled lance. Both work well, the angled one can be used underneath too but I tend to use that one for up in the arches.
Cheers,
Steve
I’ve just bought one it does clean the underneath of the my ND. I also tried in on my son’s Suzuki JImmny, and it cleaned of the mud and salt.
Thanks for the tip, just bought one of these myself and found it to work pretty well, was on promotion too on Amazon 
Atom-Mac in a garden pump sprayer after you’ve sparayed the car underneath with a hoze or whatever?
I use one of these off ebay ((£10) for under the car and the wheel arches, plenty of other uses around the car and house with the variety of nozzles provided.
…just be very careful which nozzle colour you select! 