It is best to remove all of the old paint, but if you do not, the
RustSeal will still adhere to the old paint. Just give it a light sand and make sure it is free of dirt and grease.
You can paint the entire interior. No problem with that. The brush marks flow out and it looks much better than decent when it is fully cured.
You could use the
RustSeal on your bumpers. Just make sure you topcoat any areas that will be consistently exposed to UV rays.
You can find complete and detailed instructions for RustSeal and the KBS 3 Step System here:
http://www.kbs-coatings.com/RustSeal-Information-and-Instructions_ep_38-1.html