Polyester primer is a must use product. It provides a very good foundation for the paint process; it doesn’t shrink, it’s very solid, very low solvents, it fills great, it sands great, etc. The challenge with polyester is how hard it is. It must be sanded completely flat, without any doubt.
To get it flat, genuinely flat, means hard blocks, and the correct grit. The blocks can’t have any ‘give’ to them, as in the hardness of the surface the sandpaper is on. The blocks can (and should) flex, but the surface can still be hard. This is a big reason why True Blox work so well, they offer a very hard backing to your sandpaper, which allows you to cut this primer flat. It’s true that other blocks can work, like durablocks or afs blocks, but, it’s impossible for a sanding block like that to be able to cut the material as flat. This is all about taking your work that little bit further, which is what makes the huge difference.
100 grit! This is the perfect grit to start with. It’s preferred over 80 because it cuts the polyester down just as well, but doesn’t leave behind those deep scratches. It also allows you to jump up to 220 grit after. You could never go from 80 to 220. Skipping a sanding step is huge. Also, 150 is just too fine to actually cut the polyester flat. Even though it appears flat. When it really gets down to producing exceptional work, these little differences matter.
>Fianlly, make sure you sand the polyester completely flat with 100 grit before moving on. There should be absolutely no texture left in the primer.