Joe,
The best possible procedure is to use the correct sized tape so that it covers only what you dont want to hit with the buffer, but not any painted adjoining painted panels.
This is why painting/detailing masking tape comes in sizes from 1/8", 1/4", 1/2", 3/4", and so on.
You always want to polish/clean/clay the paint as close to moldings, black plastic places as possible so it all looks the same. If you cannot do this for whatever reason, at least, carefully, apply your sealant in those places if possible, so that they are at least protected.
Sometimes, Detailers have to resort to using Q-tips to get product in a spot that is hard to get to with power tools like in between letters, numbers, emblems, that are sitting on the top of painted surfaces. We also use the same products to clean out those same areas before applying anything else.
We also remove emblems, etc., if its easy to do, to really polish underneath them.
Its all in to how much "Detail" do you want to perform to your, or a Client's vehicle.
For example, you will almost always have to polish the areas behind the door handles by hand because a buffer cannot get into the small space and if you rub too hard on the curved edge with a machine, you run the risk of cutting through the paint on that edge.
What I personally do after taping up black plastic/rubber, etc., molding I am not wanting to accidently hit with my Rotary, is use the rotary through the stages its needed, then sometimes use my Flex 3401VRG to apply the last step of polishing or jeweling the surfaces, then pull all the tape off, clean up any stray compound, etc., that might be under it, the carefully apply my LSP, either a Sealant and or a Wax, etc., then go over the black plastic/rubber moldings with the cleaner Ive selected, treat all said moldings, and then its done.
Others may do it differently, but this works great for my needs.
Yes, you can use your Flex (which Model ?) on painted rubber bumpers, or moldings, just fine; however, pleae keep in mind that these non-metal painted areas heat up much faster and dont release their heat easily because they are not metal, and the paint can mess up easier if you press really hard for too long on them.
Also, you dont really know how much paint is on these parts, and they are not typically measured for thickness with any of the standard paint thickness gauges, unless you want to spend a few thousand $ for one that measures plastics, fiberglass, etc.,as well as steel and aluminum..
Yes, you can apply your sealant, etc., with the Flex, but I hardly ever do this, and apply it all by hand.
The reason is because its easier, I have more control, and dont have to worry about hitting anything with another power tool, and you use alot less sealant, etc., to cover the whole vehicle, because you are using a relatively thin 2.5"-3" pad and it absorbs only so much, while a big polishing pad will really suck up your product.
If you used blue Painters Tape of any of the masking tapes used in Painting vehicles, it should release and not leave any residue; you should not leave it taped up for a very long time, though. A few days to maybe a week, check on it if it has to stay taped longer and make sure the tape isnt drying out.
Good Luck with your project !
Dan F