Check the project farm video on YouTube on different brands of rust converter benchmark. Plus fluid film I estimated $300/yr. I also live in rust belt but not sure how long I would keep the car (2015 model). Also fluid film doesn't cover high temp equip like the outside of catalic converter and exhaust pipes right?
There's mixed reviews on rust converter.
I used Rust-Oleum Rusty metal primer spray can for light surface rust areas.
Luckily mine ( 2018 cx5) was only light rust. Wish had watched videos first.
But there are some youtube videos which tested rust converters found the high phosphoric acid converters that are paintbrush on work best. Check vids for products. "Repair geek" video results:
1. Rust Kutter paint on
2. Corroseal paint on
3. VHT spray on
All others failed testing... See video
Of course Coke and Pepsi have phosphorous acid so guess you theoretically could make some cola gel and maybe paint that on?? Not sure how well would work. Although I was able to easily remove rust from alloy wheels with just Coke & aluminum foil, not sure if best solution for rusted frame.
Also don't spray fluid film on cats, exhaust, etc as will burn off and don't spray on shocks /strut assemblys so you can see if they leak/fail. Avoid spraying rubber, boots, etc.
Only spray the underside that's exposed ( Mazda has covers on alot), really spray the rockers, the frame around the strut/shock assemblys, wheel wells not covered by Mazda covers, etc. Don't forget to spray fluid film inside the frame holes, bolt holes, pass-thru holes, etc. Anywhere there's a hole, get some FF in there. If unsure, watch some videos on undercoating car/suv with fluid film. "Repairgeek" has good YouTube video on fluid film. Don't forget to wear your PPE respirator/mask.
Unless you want to remove all the Mazda covers and spray the whole underside ?
Was going to wait and check the car next year and if starts rusting under the Mazda covers then may start fluid film the whole thing but for now seems overkill.
Also is $300 to get it done elsewhere? $300 seems extremely high for DIY and seems high to have done at a shoppe. Only cost me approx $60 to $80 to fluid film it (uncovered areas only as the 2018 has alot of underside panel covers). Should only cost about $150 to $180 to have done by a mechanic, maybe even less. (Assuming 1 hour labor and bulk material costs). Fluid film once per year would be ideal but even once every two years would still be beneficial.
Keep in mind, mechanic can throw on high lift and spray on quickly with bulk fluid film so shouldn't take them more than 1/2 hour to 1 hour labor plus material cost. Imo, really shouldn't cost more than $150 - $180 to coat whole underside (without covers). To save $$, you could take the covers off before take in to the shoppe to get sprayed. Why have them charge $60 or $80 hr just to screw off panel covers? Also either DIY or find an indie...Don't pay dealership $150 labor costs to do something this simple...you'll end up paying dealer $1000 for a $200 job.
If you keep the car it's worth it to FF. If not, the FF treatment will at least make vehicle more presentable/ easier to sell an un-rusted frame.
When buying a used car, I looked under and passed on any car with rusted frame.
Lastly, don't go through auto car wash during winter months..it will wash the FluidFilm off the undercarriage and leave it vulnerable to the salted roads. If you must wash your car in winter, hand wash the body or use car wash stalls with wands to do body only.
You have pics of your 2015 frame? At 7 years, it's probably just starting to look bad as any factory coating has to be wore off by now. How bad is the 2015 ?