Help! I bit off more than I can chew! Body work

went with 3m light weight body filler and started on the roof seam that was under the vinyl top. used a grinder to remove the rust. filled with filler and primed. now i see my mistakes and will attempt to fix. ive been using a palm sander which is all i have. would using a sanding block be a better way to go? funds limited. i can still see where the seam is. need to build up a little more. my plan is to leave the vinyl off and paint the whole car and i want it to look right. any tips on this particular area would be great. will load a pic of this shortly.