As great as True Blox work, you still have to switch to a different block for the final sanding step. To block sand polyester, the most efficient steps that result in the highest quality is using grits 100, 220 and 400.
The purpose of the first two step is to get the primer flat and straight. The final step, 400, is to smooth everything out and create a perfect surface for sealer to lay on. For this, you have to use a soft block.
A couple different ones work great. For the bulk of the sanding, 12” long foam blocks work well. Up against tight corners, something a little more firm must be used. Below are pictures of examples. Take your time sanding up to detailed corners first, making sure not to sand grooves or lines anywhere.
