Just now seeing this or I would have commented sooner. I would suggest plexiglass sanding blocks in this situation, thin enough to flex and follow the contour. A full width block would need to be less than 1/16" to be flexible enough, or a half width block could probably be made from 1/16".
One thing I don't like about foam block is that they're too soft on the surface and won't "force" the paper to cut the high spots down. The softness acts like suspension and soaks up the high spots, so you're sanding both high and low spots at the same time- working against yourself. It can work with a lot of patience and very light pressure, but it's slow and won't give the best results. Plexiglass blocks will flex enough to follow the overall shape but the surface is hard so it only contacts and cut the high spots down. Foam blocks are too soft and can follow an irregular shape, where plexiglass bends in a natural arc so your filler or primer is shaped into a consistent radius. The sandpaper also won't "bunch up" in the middle of a curve like it does with foam blocks.
I used to use durablocks and AFS adjustable sanders but have almost 100% switched over to plexiglass after seeing how much better it works. AFS sanders are better than most since they rely on a spring steel surface which helps the paper cut only the high spots, but they still don't flex as well or as naturally as plexiglass.
I recently block sanded a pair of stepside fenders that another local bodyshop had prepped. The shape is similar to the area you're blocking. The shop pretty much quit working on the project and the owner asked if we would paint the fenders so he could finish assembling the truck (the rest was already painted). I don't usually go over another shop's work but I was curious to see what they had done, they were supposed to be "ready to wetsand and paint". When they were delivered they were in polyester primer with a lot of texture so I started blocking with 80 on plexiglass blocks to knock down the texture. The other shop must have not had any type of flexible sander... the tops of the fenders were fairly straight but the ends were horribly lumpy from being improperly sanded. You can see just how lumpy they were in the pics.
First few light passes show the high/low spots.
Heavy blocking to lower the high spots. I've already cut all the way through the poly in places, yet there is still guidecoat showing in the lows that hadn't been touched yet.
More blocking to even out the shape front to rear- not focusing on the corner as much yet. Found a high spot in the metal as shown by the black epoxy. There were still low spots at this point.
Blending the corner shape into the rest of the panel, and blocking down to the body line to shape and straighten those areas. You can see that I'm all the way through the poly, a layer of high build, and into their filler work. There was still a low spot after this much blocking. If these fenders had been blocked with plexiglass from the start there would have been much less material build up and ended up much straighter.
