"7. Apply West System 105/205 or System Three SilverTip Epoxy following instructions for application. Apply the epoxy to the blister cavities by brush making sure all blister surfaces are thoroughly saturated with epoxy resin. Let dry 3‐5 hours at 77°F. then proceed to step 8 or allow the epoxy to cure hard and then lightly scrub it with plenty of fresh water and a Scotch‐brite pad or soft scrub brush to remove any amine blush formed during curing. 8. Sand West System or System Three Epoxy with 80‐grit production paper and wash with Pettit 120 Brushing Thinner to remove the sanding residue."

https://www.westmarine.com/buy/pettit-paint--120-brushing-thinner-general-purpose--P004_121_005_505?recordNum=56

#1 - Good folks at West Marine response:

Pettit did not suggest a barrier coat. But if you will be taking all the paint off, additional barrier coat may help if you have the time and inclination. 

If you don't want to do more barrier coat, then you might get some extra help from wiping the entire area with a prep solvent as specified in the Pettit blister treatment guide. http://www.pettitpaint.com/media/3489/how-to-gelcoat-repair-and-blister-prevention-pettit-protect-1.pdf
 

#2 -A follow up discussion from West Marine & Petit:

"We suggest sanding the entire bottom with 80 grit, get down to barrier coat and remove all residue. 

Then sand bottom smooth with 220. Then apply 2 or 3 coats of FW-21. 

When it is flaking in areas, usually its just a sign that the entire bottom is going to let loose at some point. 

They should take the precaution now and prevent entire failure in season."

#3 Petit’s parent company technical team also replied directly to me:
Clean with 216 special thinner sand clean again with 216 allow 30 minutes and apply new coats. If you’re going to use Interprotect you will need to sand back to gelcoat Interprotect cannot be applied over existing paint.
 

Peters’ plan of action - stay tuned:  bought Ryobi random orbital sander ($45) & 80 & 220 grit sandpaper from Home Depot. Will be sanding and wiping with Interlux 216 solvent.  Headed by to boat over the next 2 - 3 weeks  and will send updates. (sorry about earlier edits on this topic - had ipad issues with our web site - lap top to the rescue).  (FYI - years ago, I learned the hard way that the exact thinner recommended by manufacturer is critical - had horrible paint sags when my painter on another boat used the thinner he had on hand and big sags happened.   Pay the extra few bucks using the recommended products - and hope for the best....).