I'm a big fan of the Fluid film, and now they call it Woolwax, same shit buy no odor and slightly thicker. I've been using it for 10 years on all my vehicles, and for the fleet of electric utility trucks at work. We apply with the a specific air gun made to apply it, don't pressure wash the under chassis all winter, then in the spring give the underside a good powerwashing, then repeat the following late fall. subsequent applications are fast as it is usually just touch ups. I find it is great for areas that have surface corrosion already started, it seems to really slow down the progression. I use 3m cavity wax on new vehicles, or newly installed underside components, like brake line fittings, etc. Some will bitch about the mess if you need to do a major job on the underside of the vehicle, and they're right, but before i send a truck out to have major work done we use the heated pressure washer to clean the underside first, then after we get the truck back just reapply if it is still winter, and salty roads. The mechanics seem to appreciate the effort. I have a place an hour away that I order from called
free shipping on orders over $49.50. They have the guns too. We order it by the 5 gallon pail and have found the pro gun works best, I have the standard gun at home and it still works, just not as fast. We have found that some people have a sensitivity to the product, it just irritates inside their nose, but a N95-N100 particulate mask solves the problem. Unlike the old used motor oil treatment, it does not attack rubber or plastic.
There are a few around me that shoot it as a side gig. I find it best to use a lift, but lying on your back works too, just get the vehicle as high up on blocks as you can to cut down on fatigue.
I don't use my lift or garage to apply anymore since I started on my 68 firebird project, as I don't want to chance my project getting any of the over spray on it. I can imagine that corrosion proofing and autobody work should not be done in the same shop simultaneously.
Oh and I put sheeting or cardboard on the concrete under the lift to help with easy clean up.