I have some serious scuffing on my car’s door sill. I bought some black xpel film to prevent this in the future, but I want to do something to prevent rust from forming underneath the film.
I bought some color matched touch up paint, but I’m wondering if I need to remove all of the paint and primer from the area. Since I don’t care how it’ll look under the film, I’d like to just rough it up with sandpaper, prime and paint it, and then slap the film on it.
The part I’m worried about is that it’s a fairly large and irregular scuffed area. Some bits are down to metal, some down to primer, and some still has the clearcoat. I can’t really mask it off and only get primer on bare metal and the original primer.
The reason I don’t want to sand it all down is that it’s a difficult area to sand. I’m worried I’ll fuck up the paint beyond the area that would be covered by the film.
Can I do it the lazy way, or should I just bite the bullet and sand down to metal?
It’s water based touch up paint if that matters.
If you’re just going to cover it and the actual paint condition won’t be visible, I would use Ospho to neutralize the rust and then do a light scuffing with sandpaper and put the film on. I would use Ospho because I’ve got some handy. Since you probably don’t, you could use some rust neutralizing rattle can primer after knocking any loose rust or other debris off.
You don’t need to remove paint down to bare metal in pretty much all but the most detailed, show-car level paintjobs. If you were trying to paint match and not planning to use the film, you would just need to sand until there’s no visible rust and the shine has been taken off the undamaged paint, then use a filler primer to level out the area, then light sanding, then proceed with color and then clear.
The knowledge I have is from researching this stuff for bikes and it has a lot of overlap because we use the same materials and supplies - e.g. 2k rattle cans and clear coat. So I’ve read a bunch of recommendations and videos about cars as well.
Most car people will say that if you want to prime+paint+clear coat you need to take off and redo the whole panel. Like sand it down to at least the primer and go from there for the whole panel. This also usually involves proper color matching, knowing the exact color code. Basically nothing adheres well to an intact clear coat, including primer, so the edges of any “spot fix” with your strategy will be prone to flaking, as the clear coat will be intact at the edges. Some people seem to get away with spot fixes using these supplies but they also tend to be experienced at doing these kinds of things, have airbrush setups, that kind of thing.
If it is a small section, many people use best-guess color matched nail polish as a spot fix.
If you want it to look right you need to take it to a shop.
It’ll be expensive, but they’ll get it so close that only someone who knows the car inside and out, or knows how to clock paint differences will recognize it.
If you don’t care how it looks then just scuff everything and use a rattle can.
Comedy option: precisely scrape and sand the damaged area, use glue to attach gold leaf to it then clear coat over. Beater kintsugi!
I forgot to focus on the rust part of your question.
Preventing rust from scratches is the original primer’s job. If it is fully scratched through to metal and you only care about preventing rust there’s basically two options I am aware of. The first is to refinish the panel or do an expert job at spot repair (allegedly very difficult), beginning with heavy sanding to identify where the scratches are and ideally remove them so they so not remain nuclei for rust. The other is to clean and adopt a protection routine to slow down any rusting process. Basically apply some mild anti-rust, wash thoroughly with water to remove electrolytes, dry immediately and thoroughly, perhaps with rubbing alcohol, and then wax. Consistently waxing every two weeks or so creates a protective barrier. There are easy spray on + buff waxes out there if you want to do it in just that spot.
Monitor the paint for bubbling, which indicates a deep and pervasive rust and a need to replace the entire panel.
i think (heavy emphasis) you can lightly sandpaper over primer, and if it didn’t adhere to old paint it will fall off anyways? but if it falls off after painting, it’s also no harm cause there is paint underneath 🤔
