I really want to try the underside myself, the top I will leave to a pro!

Did try dry sanding but with 600 grit it just clogged the paper instantly

Now I sand a bit and when finished fire up the fan heater and leave the halogens on to try and make sure all the water has gone from the seams.

The epoxy primer is non porous so there's no worries with micro blisters through absorbed moisture.