Sometime a while back I wrote a description of the thicknessing/bottom rocker process for PU blanks, i was hoping I could find it and paste in here - no luck. Maybe somone with better search skills than I can find it. Anyway, here's a second ''pass''....
Look at the blank first. Check for twist or any high or low spots. Before you go at the thicknessing, ''block'' the bottom with your planer near zero if needed to remove any glue drips and/or get stringer flush. A smooth surface is required for everything to go right. Then you're going to figure how many cuts you're going to take to get to desired thickness. The best way to minimize the ''trails'' and make the blocksanding very easy is to leave yourself a half pass for final cleanup on bottom, so figure your cuts accordingly. Generally, the way I was taught was to plane to within ~1/8'' of finish while thicknessing and doing rocker and bottom contours, then take that final full-length pass at 1/16'' depth. Rocker/foil changes are done with stagger or fade cuts, which is another subject.The other 1/16'' you're going to take of the deck when you get there.You barely have to blocksand the bottom if done properly (unless there's lots of concave, concave always involves a few ''trails'', lol)
If you're having trouble getting an even (full depth?) cut at nose and tail, it may be because you're used to pushing down on the tool in the middle of board and when you get to the ends the blank moves away from you; since it's hanging off the end of rack. Is the other end rising up off the rack when you do this? The solution (for me) was to get used to very minimal pressure on tool at all stages of cuts, and of course you have to pitch the planer properly to follow the curve in rocker.
The ''trails'' are left by the corners of your blades, and it's a subtle dance of pressure to avoid them. I wish I could explain it better. It's something that will come with experience. Don't worry about it much on the initial cuts, you're just going to mow them off in the next. That careful cleanup pass at the end was a key lesson for me.
If you're skinning and doing rough thickness before outline, doing what grasshopper said on first cut along rail will work. But if you've cut your outline first, there's the risk of putting in a little wiggle right in the wrong spot. To keep it flat and get that first pass clean, you have to start out with pressure on shoe (forward) but quickly shift to even or a little more on baseplate (rear) as the baseplate becomes engaged with the blank (it starts hanging off in space). I always start from tail on the perimeter cut, because that's how Jim P showed me.
girvin, you already know this, but for others reading - the perimeter (first) pass follows the outline, each succeeding pass gets straighter until you get to stringer. Stringer is done slowly, and with the grain (on bottom that's from center to ends, vice-versa on deck). Don't just ''follow the blank'', many PU blanks have little imperfections that experienced shapers will get to know. If it's your first time using a blank look it over carefully before starting and check constantly as you shape. Almost all PU blanks require some foiling at tips, otherwise you'll get a ''blob'' nose and/or tail.