Craigs advice sounds about right to me.  Keep in mind that Gelcoat is also a Polyester and only a bit better than the actual laminate in stopping moisture penetration.   It is pretty nice to seal off all of those glass fibers that will be at the surface of the laminate in places so Gelcoat makes sense there.   Epoxies are a lot better at stopping moisture but some have toxic elements that I would not want in my water tank.  The studies I have seen show that properly cured polyester seems to be pretty safe for a water tank but I glad to hear if anyone else out there has heard otherwise.  There are supposedly some drinking water safe coatings out there but I have no experience with them so will not speculate.  When it comes to blistering problems some boats get them and some never will.  For those hulls that do want to blister barrier coats only seem to slow the process.  I can only speak to the Amels that I have inspected but in general blistering seems to be rare with these boats which is a testament to the construction. Basically osmotic pressures are not strong enough to seperate the layers in a well laid up hull built with good resin.   Prepping the bare spots and gelcoating the inside of your tank would be a good thing but I would be reluctant to coat any areas that you cannot properly prepare the surface to get a good bond or you will have more flaking.  Also if you are using regular air inhibited gelcoat, you must use an addititive with your gelcoat to cause it to cure properly on the surface or it will remain forever sticky and you might smell or taste this in your water afterwards.  In the past a simple wax additive that floated to the surface of the gelcoat worked ok to seal off the air enough that the surface of the gelcoat to allow it to cure more fully on the surface.  Now there are some better additives that are commonly used in gelcoat repair that work better.  I would strongly suggest that you mix up a sample of any gelcoat that you intend to use inside the tank and do some testing on some scrap material to be sure that you have a good cure!  Nothing would be worse that having a water tank coated with something that will never fully cure and there are lots of production boats out there with sticky gel coat in the lockers, bilges etc.  One thing that will help with the cure is to remove the vapors from the gelcoat that will tend to settle to the lowest point in the hull. Use a fan or blower that will not be an ignition source.  If the vapors are concentrated enough they will tend to inhibit the cure of the gelcoat even if everything else is correct.  With regards to sanding in a very tight space you will need to be creative.  One idea that might help is to use a flexible "snake drive" on a drill or dremel tool with an abrasive disc such as a Roloc on the end.  You can attach a stick of the appropriate length for the area being sanded to the sanding disc end of the snake to control the sanding disc but take care to not cut into the laminate with the edge of the disc.  You can mitigate this problem but using a 2" roloc tool arbor with a 3" disc which has been softened up so that it will curve away from the surface to be sanded at the edges.  Our Maramu has some flaking gelcoat in the water tank too and someday I might coat it with something but it is not high on the list.  Our boat has never had blisters inside or out so I am not too concerned.

Best,

James Alton

SV Sueno

Maramu #220

Marmaris,  Turkey