OK you are now on one of my specialist subjects (although I am not bang up to date). I also had challenges with the i3 sills the products you need to restore them are bumper care products - remember those old lightly textured black bumpers we had, the sills are the same stuff. Just give them a good wash then apply the product with a sponge (decorators slightly hard sponge is best.) I use Autoglym Bumper Care or Turtle Wax Tyre and Bumper Care.
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For interior all surfaces including any wood, plastics, seat plastics (if you don’t have leather) and the Kenaf (hemp composite) I use 303 Aerospace Protectant - this produces a great finish and is really easy to use. (Using this is an old show car trick). 303 is really easy to use. Can also be used on rubbers to keep them protected and supple. Also is UV protectant (as originally designed for protecting aerospace composite.)
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For the exterior paint work, just use a standard exterior automotive sealant / polish e.g Autoglym Super Resin or Maguiars. Use the same car care approaches you would use on any other car. However the top coat is not as hard a laquer as on some vehicles (e.g. our Mercedes C Class has a ceramic paint which is very hard so takes some polishing if scratched). So I go easier on it but it is very resilient. Do hope this helps, the challenge is now I have opened up a auto detailing rabbit hole but I wanted to keep it simple. I just wash my car, dry with a micro fibre and then use the bumper gel. Then when I vacuuming it out use the 303 on a microfibres to wipe all the plastics. Every now and again I will wipe over exterior with a detailing spray protectant. No need to spend hours to keep it nice.