Oh ok thanx, I also used a da 500 on a small block, held it down flat and not too hard as I ran through the whole panel, but I think ill use a guide coat through every grit to make sure all marks are flattened. I don't want to end up with the base looking scratchy
I've got a lot of experience blocking sanding not only auto finishes, but various boat projects, aircraft projects, and all sorts of stuff. My favorite hard blocks are now approaching 50 years old, made from some scrap aerospace flooring material. It's a honeycomb core faced with aluminum on each side. I have several of them 9 inches long (the width of a piece of sand paper) and 2 5/8" wide which allows you to cut a third of a sheet and wrap it around the block, and 3/8" thick. They're hard and very flat and just right when you're trying to make something absolutely flat or make some edge straight.
Of course, most of a car body is not flat and for that kind of sanding a hard foam block is preferred since it will conform to the curved surfaces. I use the same sized block in urethane or Divinycell foam. This is a rigid foam material, not the kind of soft foam your wife uses as a yoga pad. A 3#/cuft block is good for nearly flat surfaces and a softer foam around 2#/cuft is perfect for more curved surfaces.
Many people use those small, hard, rubber blocks for sanding curved surfaces, but those blocks are really too small and way too heavy. A larger foam block which conforms to the surface is much easier to use and produces a better finish.
Aircraft Spruce and Specialty, a company catering to home built aircraft builders, is a good source for small quantities of foam. The 1" insulating foam found at your local DIY store is not ideal because the density is lower than you really want. However, a typical 1" thick sheet will be not far from ideal as far as stiffness goes. The downside is that it will dent easily. The upside is that it's cheap and you can get it locally.
In any case, give a rigid foam block a try cut a size that will take 1/3 sheet of paper.
