I’m sure the pre-wash with the baking soda helps, I just don’t know if it helps any more than a pre-wash with plain water. But there may be other stuff in the road spray that make the baking soda a good idea.
Road salt – unlike food grade table salt —probably contains a good deal of mineral impurities. Perhaps the bicarbonate helps remove those from the car’s undercarriage. I think there’s a trick to de-tarnish silver using a bicarbonate solution and aluminum foil.
We use this product in Canada.It was developed for a large rustproofing chain 'Krown"
https://www.salteliminator.com/
Here’s some more road salt trivia: When I lived in Colorado IIRC they used ammonium nitrate as part of their road salt mix. An AN solution has a lower temperature freezing point than NaCl salt. AN is also used as a fertilizer in agriculture, so isn’t as toxic to the plants and critters living along side the road. The problem with AN is that it can be weaponized. I think that was the main component of the Oklahoma City terrorist explosion. My guess is they’ve stopped using AN for road salt in Colorado.
Around here, last I checked, road salt was largely calcium chloride, not NaCl. Some places use magnesium chloride. Others use a mix of all 3. Still others have been playing with de-icers made from things like corn or beet juice, which I wish would become more widely adopted because beets don’t usually rust cars. 
When you say beet juice I presume you are talking about sugar beet’s I could think about better use’ for corn juice.
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Yeah, but Blanton’s costs more per ounce than road brine.
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Long ago someone recommended using linseed oil as a rust preventative/sealer on tools and it does seem to help even over pitted rust spots. What is in the professional product?
Dagosa said he sprayed oil under his vehicles when he bought them and repeated the process periodically. He also said he wasn’t the only Mainer that did it.
the first car I had in my hands for repairs was from my father who used oil to “protect” the underbody
it was OK-ish on the look, but under that it was all rusted, as once water+salt was getting under the oil, it would not escape, so it is not only bad for environment, but is not panacea either
You can get it washed a lot of times for the price of the undercoating . The best thing you can do i to keep it washed and waxed if you live where they salt heavily .
Heading to South Dakota a year ago, it was icy and I pulled off on the ramp for a bit, and then came a huge MNDot semi tanker truck. He sprayed two lanes at a time. No idea what it was but it took care of the ice in a hurry. Never walked on it so don’t know if it would have eaten my shoes or not. Rolled along pretty fast too. I suppose the driver was Driver 3 or 4 class, not a run of the mill plow driver.
What type of undercoat? If it is the oil spray type like the type mentioned in the article that George_San_Jose1 linked in post #4 above, that probably would be a good idea. That requires a yearly application.
If it is rubberized undercoating, I would avoid it. I had that type applied to a new car once. At first, it was great. But I kept the car for a long time. The coating lost flexibility as it aged, developing cracks, which allowed moisture and salt to get between the undercoating the vehicle’s metal. It ate the car alive.
A friend of mine who also lives where the roads are salted in the winter recommended that I come to grips with the reality that rust prevention is a lost cause, and do what he does, instead. He gets a new car every three or four years and lets the next owner(s) worry about rust.
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