Not being a native here I was naive to how much damage the winter driving conditions do to your car. I had a truck that rusted pretty bad that I just sold. Do you guys undercoat and rust protect your newer cars? I’ve read that newer cars are less susceptible to rusting and also that just washing them regularly in the winter takes care of all of it as well. Do you guys do anything to your newer cars or do you just wash them during the winter?

You can get a rust preventative coating sprayed underneath every fall, they seem to help on the newer vehicles. The old thick rust proofing seems to trap water and salt as it cracks and peels, causing more rust.

Spay on thick coatings will contrary to their claims will rust out your car faster check out the link and make your own decision https://www.youtube.com/watch?v=nXvl9nt57Kg&t=451s  I have a 2008 taco with very little rust and wash it every winter about every three to four weeks and spray the hell out of the frame at the do it your self car wash it's cheap and effective.

I've been using Fluid Film since 2012 and it works great. Coated my wife's 2012 F150 the day we drove it home from the lot and when we traded it off 6yrs and 84000 miles later it was almost spotless, the only areas that had any surface rust was around the suspension components where it's hard to get coated. The sheet metal including the insides of the rockers was spotless. I did the same thing when I bought my 2013 F250 and when I traded it off in 2019 with 75000 miles it looked like it just came off the showroom floor, only rust on it was the axles(didn't coat them the first year but did when the surface rust started showing)and the parts that come pre rusted from the factory which are the steering box and tie rod ends. Even the spare tire wheel which catches all the debris and will rust within a year was spotless.

I've been doing the same thing to the 2018 F150 and 2019 F250 that we currently have. The bodies probably don't need the coating since they are aluminum but I coat them anyway. I apply the fluid film twice a year, once in the spring after the salt is off the roads and again in the fall before it starts staying below 60* because I think the fluid film flows into the cracks better when it's warmer. Some guys only do one application in the fall but I like to keep the truck coated year round. Then after application I drive down gravel roads to get a layer of dust on it to help it from getting washed off as easy. Outside of the two applications I do not wash the underside of the truck with anything more than a quick low pressure swipe to get any built up mud or heavy salt residue off because high pressure will blast the fluid film off with it.

I have modified my routine on application a bit. Used to powerwash and reapply fresh twice a year but last year I decided to leave the spring application on and just spray over it with some fresh to build up the thin spots from driving in the rain.

It's true that Fluid film will soften some types of rubber. It will ruin the rubber gaskets that Ford puts at the bottom of the doors and I try to keep it off the brake hoses and the vacuum lines going to the IWE hubs on the F150 since I don't know how they will react. I've not had any issues with any of the other rubber or plastic parts on the truck.

Only other thing I can think of is that if you are picky and want the visible parts of your vehicle nice and clean looking you will not have that with Fluid Film. Everything that is coated will have a grimy nasty coating on it but the metal underneath will be protected, I struggled with that at first but my wife made a good comment about that that changed my mind- "would you rather have the grimy part with no rust underneath or a clean rusty part?".

I buy my fluid film from the Rust Store and get it in the bulk(the regular formula, not the liquid A that's for indoor use only) because the bulk stuff is thicker and holds up better in high spray areas. I use the aerosol version for inside the rockers, doors and anywhere else that doesn't see water spray and is hard to get to. I have one of the Fluid Film spray guns for the bulk and I bought an extension hose with a 360* tip for the aerosol cans. you can get the bulk spray gun from the Rust Store or on Amazon(these will be marked with Woolwax which is about the same stuff as fluid film)but I can't remember where I got the hose for the cans.

If you buy a used vehicle that has surface rust started the Fluid Film will slow it down and prevent new from starting.

If you choose not to coat I have started to buy into the theory that washing too much can be just as bad as never washing. I had an 01 F150 that I bought when it was two years old and was spotless, I washed that thing every single week and it started showing rust quicker than the previous truck that I would go all winter without washing. I think the water got into the nooks and crannies from the weekly washes and those areas never had a chance to totally dry out and accelerated the rust. My dad never washes his trucks(the rain and snow washes them)and even though the paint ends up faded and trashed he has less rust than a vehicle of the same vintage that gets washed, he still has an old '95 F150 sitting out back that doesn't even have the wheel wells or rockers rotted through yet.

I'll second Fluid Film and the twice yearly applications.

Buddy  bought a used Super Duty a few years ago that had the frame sprayed with Line-X by the previous owner. I don't know what it cost but I'm thinking it was very expensive to do. When I bought my 2019 I had Line-X spray the rear wheelwells, the area that was going to be underneath the Bushwacker fender flares(aftermarket flares are kind of hard on the paint behind them) and also had them spray the area in front of the rear tires to create a rock guard like what is behind the rear tires, all this cost me around $500.

I'd be curious to know what it would cost to do the underside of the truck but I'm thinking you'll get sticker shock. IMO the only way to properly line-x the underside of the truck would be to pull the box and cab off to give good access to the frame. Also, there are a lot of brackets and other things bolted to the frame/body and you'll have to disassemble them to do it right, if you spray over everything you'll have to grind the liner off if you ever want to remove the piece.  Not saying it couldn't be done but it will be pretty labor intensive and I'd want to make sure the guy tearing my new truck apart was going to get it back together the right way. If I was rebuilding a wrecked truck or restoring an old one and the frame was going to be stripped then I'd go for it, on a new truck I'd save my money.

Another thing I'd be worried about is that if the liner got a gouge or chip in it that went to the metal then it creates a path for moisture to attack the metal and the next thing you know you have rust forming under the liner and major problems. Same thing can happen with the rubberized undercoatings and the other types of rust proofing that dries. I know Line-X is tough but it's always a possibility if something large hits the coating.

I still stand by using the Fluid Film if you can live with the underside of the truck being grimy all the time. If you need to work on it you can always wash the grime off and then reapply it afterwards. Fluid Film is also now offered with a black tint that would look great on a vehicle that doesn't see gravel, if you drive gravel save your money on the black version because it'll get a coating of dust on it that will turn the coated part brown anyway. I go overboard on coating our trucks and it costs me about $200/year to coat two trucks, this is two gallons of the bulk @ $40/gallon and about 12 cans of the aerosol version @ $10/can that I use inside the door/rockers and hard to reach areas.

i have used every type of spray on coating. and many dealer/commercial places

none have stood up.  every one eventually got rust under it and peeled off-- looking worse than spots that never had it

the ONLY thing that i have seen work is oil undercoat


drawbacks-- oily mess under your vehicle-- and has to be reapplied

i used NH oil undercoat (black)   you can see videos and info on the interweb.  it dosent hurt rubber either
their black version turns rusty metal black-- looks brand new
i just got a 5 gal bucket of it for 250-- will do several coats.
just need a paint sprayer to blast it on.  i bought one they had on their site with wands and such-- gets all the nooks and crannies.
(bought the same sprayer on amazon canada for like half price of NH site)