If you are determined to reuse the original skin then yes you will need to use a decent epoxy primer and if you are not welding it ie adding some spot or plug welds then i would topcoat everything on the inside of the skin before fitting. Then fit the skin, seam seal the edges and paint.
Judging by your pictures i would be inclined to buy a new boot skin but as you are some way away from the UK and shipping may be expensive then if the skin is not too badly pitted you may get away with fitting the old one back on as you say you have had it blasted.
A good quality second hand one would be another option BUT and its a big BUT it may still be hiding rust under the internal bracing and where the lip has been folded.