Thanks for the affirmation Brian and I agree, keeping paper sharp is one of the keys to getting a panel even and straight. When you block on filler or 2 part putty on a panel this size you can start with a paper as course as 80 grit. Use guide coat on your filler and or putty as well. With minimal pressure on a block (I would use a long board, 16 1/2 X 2 3/4 and gently start sanding. Once the panel is relatively straight, re guide coat the panel and move up to 180 grit on the long board. Never jump more than 100 grit on body work when moving to a finer paper or the foot print left by the coarser paper will still be there. Feel the panel with the palm of your hand to ensure that it is straight (a little trick that works for me is to turn your eyes away from the panel...if you look at it when your checking, often your eyes will lie to you and tell your hand that the panel is straight). After 180 grit, I would guide coat again, I like to move to an even finer grit like 280 grit...or in some cases even with 320 grit for my body work, finish block sanding. The reason is that when I put my primer over top of 280 grit or 320 grit it has less chance of sinking into the scratches compared to 180 grit. Prime the panel allowing proper flash times between coats and give the panel 2 to 3 medium wet coats of primer. Let the primer cure properly, guide coat again, get your long block out and gently block the panel with 400 or 600 grit paper. If you have any low spots seen by the guide coat, you could possibly fill them by spot priming (depending on how deep they are) or put on a light skin of 2 part putty and re-spot prime...block out again with 400 or 600 grit...clean it up and paint it.
Damn that sound easy...LOL...it's not, it takes patience and practice...the good news is that with experience it does get easier.
Hope this helps.