Frankly, I see no point in sealer in your case. Your last coat before color can be primer. If you've blocked sanded everything properly you should be able to lay down a coat of primer and wet sand it smooth without cutting through.
Another thing is that "scuffing" with 600 grit isn't going to do anything. That grit is too fine. You should never sand without a block of some sort. Your final sand before top coat should be a thorough block sanding with 400 grit, wet, using a guide coat. If you do this, you will have a glass smooth surface with just enough "tooth" for good adhesion.
Remember that you top coat is only going to be as good as the work that went on under it.
IMO, of course.
