TOPIC: Sealer on before primer? & Easy blistering quirey

I took notes from past threads here, elsewhere and youtube videos about crazing and blistering and know what I am getting myself into BUT have some basic questions I can't find answers to.

1) The picture is of my hull. All the Batboat latex paint is stripped off now and I am ready to start sanding the gelcoat with 80 and 220 grit. Inside the yellow lines is where the majority of the blistering is (same areas where soggy, waterlogged foam was inside). Crazing and blistering is really bad near the stern.

Most of the delamination is visible but how do I get all of the hidden blisters to reveal themselves? Will my sanding the gelcoat with 80 and 220 grit pop them out or is there another more efficient technique? Or it is better to remove all gelcoat in those areas down to glass?

2) Here and on youtube I found mentions of applying a "sealer" on the gelcoat before primer but nothing specific.

Please elaborate.

Will be using 3 sanded coats of Easypoxy White Undercoater as primer. Is that it?

Thanks in advance.