They look lovely
.
You need to apply a UV block so yes, some car paintwork sealant is needed.
I currently use Britemax Extreme Elements on the ones I do at the Nano's but there's loads to choose from.
Even then you'll probably be looking at doing them again in a few years time. The first ones I did are about 3 years old and are showing signs of needing to be done again.
For a more durable fix you could go down the plastic film route instead.
Good Luck.