I have always wax-oiled all my cars underneath using Finigans which you can buy from most car shops, never had any rust issues despite not having a garage and also apart from my current ST all my cars were never new. My current new ST, could have had garage do it before I picked it up but it does already have a factory coating so not needed. However I have purchased from car shop a Hammerite aerosol spray wax-oil (clear) and have given the underneath two lights sprays just about everywhere, it's fairly thin so isn't too messy and won't pick up too much muck, I plan to do this a few times a year. My tips are keep it washed and polished and always run the car even at weekends if normally stands still. Also I have blocked a few "gaps" in the engine bay where water/spray/muck is thrown into the engine bay under wheel arches mainly the right one next to washer bottle but do both, your engine bay will stay far cleaner beleive me. I used "Dum Dum" to do this.
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MR200 Stage 3 200PS
Evans Halshaw
Mountune Pro