This was the full reply that Mike Whitham sent to me today...
Instructions on how to repair damaged hull are as follows;
Endeavour to dry the damp plywood as much as possible with a heat gun, being careful not to damage the gelcoat.
Apply a coat of G4 Pond Sealer.After 24 hours, repeat this process then leave to dry for another 24 hours.
Apply Crystic Bonding Paste(90-78pa) filling the damaged area to just below surface of the hull, using a spatulla or spreader.
This product requires a catalyst.(MEKP50 or equivalent) at 2% volume.If the repair is deep, you may need more than
one application,allowing each layer to cure before applying the next.
Once you have filled the repair to the correct level, once this has set, you can buff it smooth(ish).also the surrounding area will require buffing to allow the fibreglass to stick.
Before applying the fibreglass clean the area with Acetone.
Mask out the area,ensuring that the masking overlaps the buffed area.
Using an appropriate Polyester Resin & Catalyst (MEKP50-2%) apply ,firstly a fibreglass tissue then 2 layers of 450 gramme chopped strand mat & to finish, another tissue,try to avoid overlapping the masking.
Allow to cure for 24 hours.
Remove the masking & buff smooth.
To finish off, you will need to apply a TOPCOAT, using the same ratio catalyst as before.
Your GRP supplier should be able to supply you with a suitable pigmented Topcoat.