@Whiskey
Thank you for the time spent requesting additional information.
Most sailors are aware of this but should be mentioned. Never cut a thru hull on a curved surface. Why? The exterior flange of the thru hull will not seat properly and I have seen too many do that stunt gooping on a lot of sealant. Always choose a flat surface for outside. Sometimes the interior fiberglass may not be smooth sitting and the use of marine ply or something whether metal, plastic or even what I call a rubber ring to lock that thru hull into place inside on a curved inside hull. I have seen some grind out the hull some inside and discourage that. . I would encourage the use of the long cure 5200 on application below the water line, Not butyl tape, 4200 or silicon. You do not want that thru hull to move unless you want a leak.
First, clean off all the old caulk. It would appear looking on the inside below the sink in the Head area, it was actually grinded out to some degree to try to get the outer flange to sit as flat as possible. I am not sure but will address this first in two ways.
Now, I would use a Dremel with a rotating cutting edge piece to cut a minimum 45 degree edge from the inside to the outside leaving the inside diameter of the hole the same size forming a connacle shape being larger on the exterior of the hull. You can make it a sharper angle which is better again from the inside of the hull with the exterior of the of the hole larger on the exterior of the hull. Why? It gives more surface which to attach new fiberglass from the exterior. Think of it this way. You have water pressure pushing in on a hull below the water line which this hole is and with a conical shape outward, a plug for example will not push in or fail. I have seen in my years of a few where attempts were made without the conical shape and boats had to be retrieved quickly when the plug did not hold due to the original wall of the hole was left in place vs. a conical wall. Once done, then sand for a clean surface.
I would suggest the use of vinyl ester resin when below the water line. Also purchase a small resin roller I think from Seadog. When using the resin roller, have a jar of acetone ready to clean it often so build up of resin will not occur. Wear gloves.
Inside the boat, I would sand the area around the hole at least one inch from it. If the fiberglass edge has been grinded out along the hole, then we can use that and of course lightly sand it. That being the case, clean with acetone or alcohol. Again from the inside photo if the glass has been grounded out, I would cut two sections of fiberglass to fit the entire circumfrence of that hole and the grounded in portion as seen in the photo. I would then get some stiff cardboard and prebend it to shape with the hull with the two layers of fiberglass stapled to the cardboard. Also a string in it so you can hold it from the outside while resin is kicking faster. I would kick the hardener faster by adding a little more applying to the inside sanded area and then of course to the two layers of fiberglass so will set up faster. Make sure the cloth comes in contact with the sanded surfaces and precision is not necessary here. What you are doing is applying a surface when dry so you apply fiberglass on
the outside of the hull while at the same time providing a locking ring inside.
If the inside area of the hull was not sanded or grinded out, then I would sand one inch around the hole after cutting the conical shape,, cut a piece of cardboard to cover that area to include the hole and sanded area, staple two layers of fiberglass to the prebent cardboard with a string in it. I would kick the resin adding a little more hardner but not much first coating the area mentioned and then the glass. You could hold the cardboard in place or with a second person holding the string outside and it takes about 10-15 minutes. Thus you have when dry something to attach fiberglass to plus a locking ring.
FOR THOSE OF YOU WHO HAVE HOLES IN THE HULLS WITHOUT INSPECTION PORTS or you cannot get to the back, YOU CAN USE THE ABOVE METHOD. SIMPLY TAKE THE CARDBOARD (PREBENT FOR CURVED HULLS LIKE CATAMARANS) OR YOU CAN USE CHICKEN WIRE INSERTING IT INTO THE HOLE WITH STRING HOLDING IT ATTACHED UNTIL THE RESIN STARTS TO HOLD. Old timers use to do this little trick.
I would then sand lightly that fiberglass to attach other fiberglass to. Cut the first piece of fiberglass to match the size of the hole and the second a littler larger to fill the conical shape. You do not want gull wings flapping outside of that conical hole. Also between each layer, the use of a resin roller will help to smooth out the fiberglass and getting rid of air bubbles. Clean constantly with acetone your resin roller and exterior of the hole line. If on a vertical surface outside, I suggest laying two layers only as any additional layers will cause it to fall off. Allow it to dry between each application and in warm temps. If during the winter in a heated place, make sure you are vented for your own safety and health. The use of gloves is suggested. Any discarded resin with hardener, please put outside in a container so if in the event it was kicked too fast, any fire will be contained. Sand again between each layer.
Then cover the area around the repairs and lightly spray black or grey if you feel that you have achieved fiberglassing the hole in. Sand smooth. Any paint left will show any valleys and holes which to fill. If you feel that enough fiberglass has been done and need to fill, the use of two part epoxy as a filler can be used here. Once cured, I would wet sand. At this point you can spray gel coat but that is a different subject, spray Krylon plastic paint or four or more coats of epoxy barrier first over the repaired area, then the second a little further out and so on to distribute the contour in a sense of the repaired area. I would add the epoxy barrier. That is the owner preference. Then apply bottom paint but suggest lightly and I mean lightly the area to be covered.
I know this is lengthy but I prefer to speak in layman's terms as many can follow this. I did this approach based on what I read but from the wise and experience old Sailors. I am not the expert but it worked for me.
@Whiskey Please send me a private message with location and phone number as it is now time to talk. I would then ask that you report back the results as you move along with the repairs with photos. Others will chime in as many are experienced.
As for finishing off inside, that will be continued but lets go to this point for now.
