Two concerns:
1.  Is it just the resin that is being attacked or the glass being attacked as well.  With an unknown chemical it could be NaOH which would work to dissolve the glass, or HCl which could cause stress corrosion cracking.  This can not be determined without examining the glass under an SEM.
2.  How much material must be removed to get a good peel test?  How do you verify that all material that is removed has been replaced to a sufficient degree that structurally the vessel is fit for the intended service.

The end user should consider a accoustic emision evaluation prior to and after the repairs.

gvanbeek,
I much appreciate your interest in our problem. You have refered to two items I never heard before and please advise further.   
1) What is an SEM   
2)also what is an accoustic emmision evaluation.

As it happens we are now about to commence some work on the Tanker after setting up a powerful  dust extraction system ready for surface preparation either by a grinder or by very careful shot blasting to remove all degraded surface material. Thanks in anticipation of soonest reply. David W

Corrosionman:

SEM - Scanning electron microscopy.  It allows you to look at surface that has topography.  The magnification needed to look at the glass is in the range of 600x to 1200x, which can not be done with optical microscopy without polishing the surface into a very flat plane, which destroys the fracture surfaces.

AE - If you have ever heard the cracking sound that occurs in FRP when it is overloaded (such as over torqueing a flange), then you can appreciate this method, which detects minor breaking at levels imperceptible to humans.

Take at look at ASTM E1067-01.  (http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/E1067.htm?E+mystore)

Here are a couple of other resources:

http://www.pacndt.com/index.aspx?go=technologies&;focus=Acoustic%20Emission.htm

http://www.appliedinspection.com/typicalae.htm

gvanbeek

Thanks for input. I now realise what is meant by AE.  I met it some time ago without the name.  We suggested it to the Client but it is expensive and also we now find that although the gelcoat is severely degraded the glass behind it is in good condition.  A couple of peel tests gave excelent bonding results so we plan to complete removal of all gel then simply recoat. The finished job will be subject to a routine pressure test before return to service,
I thing it is 60 psi or thereabouts.
The references you suggested are very usefull although   
I have heard mixed reports on the value of AE tests.
Many thanks .   DW    

Corrosionman:

AE must be performed by an experienced, qualified, & certified inspector.  I am not sure what was the cause for the mixed results, but the fact that AE is a mandatory inspection technique for ASME Section X code vessels.

I think that the fact that you presented AE as an option for proof testing, and was rejected by your client will give you some protection.

Good luck on your project.

Gerald

Gerald