If It's a long lasting sealant the car will probably need stripping of any polish, wax or glaze so your applying it to the paint rather than the polish. This means any swirls or scratches will be even more noticable as there won't be any fillers in them.
That's the case with C quartz anyway and you have to have the correct temperature to apply it, leave it to cure for the correct time and then not get it wet for a certain amount of time. A right ball ache so I went back to using a filling glaze and wax.