Spray-on oil products like Krown or Fluid Film do not dry, chip, or crack. They remain oily. The yearly reapplication is recommended because since those products do not dry and seal on to the surface of the vehicle, they will get washed off by undercarriage road-spray. And because they remain oily, they collect dirt.
Letâs think about this for a moment âŠ
âthey will get washed off by undercarriage road-sprayâ - Assuming that it provides some initial protection, if it gets âwashed offâ when will that protction end? Ten seconds before annual reapplication?
âAnd because they remain oily, they collect dirtâ - The last thing Iâd like clinging to my car is road âdirtâ, containing an exotic mix of chemicals, road salt and moisture, especially if itâs going to be âwashed offâ to collect into those rust prone crevices that exist on all cars.
A $100 bucks wonât break the bank but I think youâll do better spending it in regular undercarriage flushes and touch up pain. .
There are probably multiple variables. Some vehicles are more prone to rusting. Another factor is the length of ownership and the number of miles driven, especially in winter. My guess is most new vehicles will not show signs of rust for seven or eight years, so the flushing would be sufficient. But I keep my vehicles for 15 plus years. I previously rinsed the undercarriages thoroughly throughout the winter. I have had better luck slowing/preventing rust using Fluid Film.
Anymore I feel the best tactic would be to buy new vehicles, wash them and rinse the undercarriage regularly, and keep them no longer than ten years. Or relocate to an area with a climate where road salt and brine is not needed.
kcac_1_157641 Or relocate to an area with a climate where road salt and brine is not needed.
Best idea I have seen in this discussion so far. 
Thereâs two schools of thought on that. One says wash all the salt off. The other says that does more harm than good. Salt doesnât rust anything until water gets involved. If youâve got a bunch of salt on your car, but itâs very cold, so the roads are dry and there arenât any puddles of water around, it may be better to just leave the salt alone until the weather warms up enough that the car will get wet from puddles or snow melt. That line of thinking also points out that a high pressure jet of water from a car wash might force salt (and water) into areas it usually wouldnât get to, where it then sits, nice and wet, and starts working on rusting the car out.
Honestly, I find both arguments have merit.
As soon as that becomes a possibility, I guarantee thatâs what Iâm going to do!
That sounds like what I did - pushing the salt into every little crevice, with the high-pressure wands at self-service car washes.
I dont agree. if you leave the salt on when the snow melts or you go through a puddle the salt will get wet and start causing damage. wash it off the best you can. the less salt the better. any time I use to go fishing on the beach with 4x4 I would wash the underside to get the sand and salt off. if you pushing the salt into crevices you are also pushing water into the same area to get the salt out.
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Hopefully, someday someone will find a way to keep the roads clear that does not corrode vehicles and roadways. Those of us in the road salt areas of the world probably spend more on personal transportation due to rust prevention, repair, or a quicker vehicle replacement cycle than drivers whose roads never see snow or ice.