there is many ways to do this. do it the way you want. but if you sort cut and try to fill 80 grit it will pop up this way .i have told you that will stands the test of time been doing all my good paint jobs this way for over 25 years .you can dump the primer on it and d.a it flat with 220 to 320 and seal it and paint it. if you guild coat you do want to mist it on and use black lacquer in a spray can if you can find it you do not paint the black on just a very very light coat or it will plug up the paper and that is why you wet sand