I'm a northeastern dweller, so this conversation is naturally coming from a rust-prevention standpoint. Not that the D cars will see salty winter roads, but will see plenty of rain I'm sure.
As the weather warms, I plan on diving back into my S13 project. Last fall I completed the cutting and welding of much new metal to perform a rust delete. As of now the new metal is in, seam sealed, and primed. I'd like to at least coat my wheel wells and forward sections of frame rail with a permanent solution.
I'd like to hold an open discussion regarding products and practices. Body and paint are another language to me, I'm all nuts and bolts. Looking to nail down a reasonable but not counterproductive approach. Things I'd like to address are:
"Undercoat" vs "Bed liner" vs POR-15. What is the go-to? Why?
Brush on vs spray on
Apply over existing undercoating? strip to bare metal? If so, via wire wheel or chemical? Other surface preparation considerations?
Luckily since it's underbody I'm not worried about prep perfection as things like runs, orange peel, etc. aren't overly a concern in comparison to a paint job. I am just overly paranoid about applying a coating that "locks rust in" or allows the chassis to rust from the inside out, as I've heard so much about.
I wire wheeled my cars spray on undercoating that the previous owner put on, self etching primered it, then put on rollbar and chassis paint. After that I applied seam sealer to the necessary spots and used fluid film in the frame rails and other areas. I’ll re apply fluid film yearly.
A light wire wheel cleaned off the old stuff and scuffed the OEM paint. If you have rusty areas go ahead and go to metal. I also SCRUBBED chassis degreaser using a med stiff plastic bristle brush and wiped dry with 70/30 isopropyl alcohol. I think this is the most important step and regardless of how you sand, be sure it’s spotless before applying whatever you decide.
What I’ve gathered from my research is rubberized coatings just hold moisture in and it’ll rot while “looking okay” and POR 15 is too hard and cracks, which eventually lets moisture in and you have the same problem. Essentially they are both just well marketed ways of hiding a problem but not actually fixing it.
Truck bed liner idk if it’s worth the cost. It that seems like the best solution but it’s also heavy and harder to clean with it being a rough surface.
A lot of off-road guys swear by painting and fluid film because it’s cheap and easy to fix when it eventually get banged or scrapped against. Being a drift car I’m assuming it’ll see some wear and tear and figured if it works for them then it will work for me.
OH ALSO, I applied clear coat over the wheel well paint to hopefully help with cleaning down the road.
I have no personal experience, but I did find this Juicebox video somewhat informative as far as knowing what the correct procedure would be (skip to 4:30) plus some entertainment value haha