Here's a How To For Keeping the truck clean and salt free in the winter. I originally posted this up on the Blue Room..
Here's what I recommend. Find a local car wash with "Do-It-Yourself" bays that spray HOT water. Hot water is an absolute must! Don't settle for less. The one I frequent will give you 14 minutes for $5.00. A pretty good deal IMO. I'm not concerned with a streak or spot free result, what I'm concerned with is getting the salt, filth off and leaving it at that. Summer is the time for hand washing and waxing and making your FJ look pretty. So here's what I do.
I wait for a day where the temps are above freezing (preferably 35+) and head to the car-wash. I pull in and before getting out I pop the hood. I then make sure my windows are up and get out and close the door. I select HIGH PRESSURE HOT SOAP and begin by cleaning out the engine bay. Once that is completed I start with the wheel wells and work my way around. You'll get better at this but what you want to pay attention to are the areas where you can get build up. There's a lip on the underside of the fender flare that will fill up and unless you spray up and back, you won't clean it out. I spend about 3 minutes per wheel well or until the water runs clear. I then crouch down and hold the sprayer upside down to begin attacking the undercarriage (Due to the force of the spray I actually rest the top side of the gun on the ground and use it as a pivot to control the spray. This will make more sense when you get to this point). I basically start at the front and work my way around. Pay attention to the backside of your wheels and if you have the rock rails, give those a good blast as they tend to hold a lot of road grime. I also spray the lower portions of the doors where road spray works its way in. If you have the hitch, remove the rubber cap, spray it out and spray through the hitch. A lot of people will forget this and they'll wonder why their hitch starts rusting. At this point I'll step back and spray down the upper portion of the truck. Like I said earlier this is just meant to remove any salt or dirt, I'm not expecting her to shine afterward but I'd rather have spots than salt or dirt. Once I'm satisfied that I've removed as much as possible (usually takes about 12-14 minutes) I switch to HIGH PRESSURE HOT RINSE and once again do a full walk around. Starting with the engine bay, I rinse it out completely and then close the hood. I rinse the hood out as there will be some soap hiding up there and then begin to rinse the wheel wells and the undercarriage. Getting all the soap off isn't crucial but I give it a pretty thorough rinse. Depending on the severity of the filth, at this point I might toss another buck or two in for an additional few minutes to make sure I've done as best as I can.
Being that's December in Idaho, I realize that she won't even make it home without getting a little dirty, but that's just the reality of the season and I see this as a relatively cheap insurance against the elements of winter. I for one, (as an owner of an original FJ40) appreciate the costly toll that winter can take on a vehicle and for $5 a week my undercarriage still looks cherry and that makes me smile!
Most people never even consider the undercarriage when washing a car, and most buyers will never even look at it. But to me, the sides and top are just a cosmetic shell that makes up only a portion of the vehicle and while keeping it clean is important, I would argue that the undercarriage is equally if not more important. If you work on your own vehicles you'll really appreciate it, and if you don't, whoever does will. If you are planning to resell in a few years and the potential buyer notices your sparking undercarriage, this will go a long way in assuring him/her of your conscientious upkeep of your truck. But even if you never plan to sell, you'll be glad you aren't spending hundreds if not thousands on costly repairs as a result of rust damage sometime down the road.
Well all this talk has me aching to go out and polish up my chassis so I'm off. But if you want a more detailed instructional or "how-to," I'm sure my girlfriend would love to video tape my process next time I go to the car wash. She's amazed at my attention to detail and is almost consumed with arousal while in the presence of my sparkling undercarriage.