Let's suppose he only wants to add the coating, not caring if there are some marring/swirls underneath. Then I'm thinking Prep-all used by body shops would probably remove the sealant completely and leave the surface ready for the coating. Of course claying would still be a must and iron-x probably also a good idea. The clay process might add marring to the paint, in addition to whatever may be there already, but if you don't have a polisher and don't want to deal with that, the wax and grease remover might just be enough to remove the sealant. The solutions we normally use aren't strong enough for that but maybe a body shop level product could do it.
Just a thought, maybe others want to chime in on this idea in case this is your preferred route.
Are you opposed to having someone professionally do it? We know it's much more money but if you end up going the direction most of us here do, you'd end up buying a lot of stuff and to do it on just this car it might not be worth the investment.
