Thanks for all the replies guys, recently I painted a car in which I had done some bondo work on it, I used Feather Fill G2 for filling and leveling, after blocking that and putting a second coat on, I sanded it smooth with 600 grit, it looked really smooth in fact that I was tempted to apply basecoat over it, which I did. (after checking the tech sheet and reading some helpful info about G2 in here of course), the job turned out to be very good, no problems with base over G2 whatsoever.
I was asking about sealer because this time around its my car the one I will
paint and was wondering if there was any real advantage to using it, if it
would make the finish look better, but if Im careful enough with my prep
work the 2K surfacer will be good to go.
Thanks again =).