Let's say you choose a ceramic coating such as Carpro CQuartz, or CQuartz UK 3.0.
Or let's say another similar durable Coating product of your personal choice.
Your mileage might vary dependent on just how clean the paint is prior to coating, but you might find you'll need to do a full decon, and possible use of a clay bar, or clay bar substitute.
Then in basically any instance, even if you never applied a protectant at all, you'll still likely need to do a paint prep step just before applying the coating.
In Carpro's line-up of products, that product would be Carpro Eraser.
Not sure of your experience level, or what tools and other products you have on hand, the clay bar processes "might" leave some minor light sleeks and marring in the paint. Note how I say "might".
That many detailers might then be inclined to then polish the paint with a D/A Polisher, very light aggressive foam pads, and a very fine finishing polish.
If you decide to use clay, then I might suggest a very mild-gentle Clay, and commonly the mildest you'll find would be Pinnacle Blue Ultra Poly Clay. Be sure to use copious amounts of a good Clay Lubricant.
Yeah I know, lots of extra steps, time and money spent, but when using these durable coatings, there's basically nothing that can be done to improve the condition of the paint after the coating is applied.
The "Plastic Baggie Test" will help be your guide to check the condition of the paint surface. The hand is placed within a very thin Plastic Sandwich Bag, which dramatically enhances your feel of the paint surface, and can better judge what, if any contaminants are adhered to your paint. Can also be used to check all outer glass as well.
Hope this helps.