Looks like some bad luck there, but could have been worse.
I have done some heavy repairs on boats including an entrire transom rebuild on my old Maxum and various other 'patch ups'! I am also a Licenced Aircraft Engineer with training on composites, so hopefully I speak with some authority, you can make up your own mind!
Put simply, there are two types of resin systems. Polyester systems are used for casting, they are cheap and they are what boats are made from. Polyester resin is tough, but its adhesion properties are not as good as epoxy. Epoxy is more impact resistant, more expensive and it has better adhesion. On the face of it, epoxy based systems are better for this type of repair. If you are going to paint over it with an antifoul system for example, then I would use epoxy.
However, if you intend doing a repair and finishing with a gelcoat, then you have to use polyester based system because gelcoat is polyester based. Rule of thumb is that epoxy will cure over polyester, but polyester systems (gelcoat included) will not cure properly over epoxy (I found out the hard way!).
Three very important rules:
1. Ensure the area is dry - cannot stress this enough
2. Degrease with Methyl ethyl Ketone or similar (assists adhesion and primes surface)
3. Do the repair in a dry environment, moisture is easily absorbed into the repair in humid conditions which will affect the resin and glass strength properties.
If I were you, I would use a polyester filler to fill the void, sand flush, key the surrounding suface, then use two-three thin plies of 'doubler' with strands (warp and weft) orientated at 45 degrees to each other (Do not use chopped mat for this, use good quality fibreglass), thus giving you strength in two planes. If you use polyester based system, you should be ok as long as you prep well. Once the repair is cured, ensure that the glass is well 'wetted' no dry strands, then lightly sand to give a finish that is almost flush or flush. You are then ready to cover the repair with colour matched gelcoat. Gelcoat is painted inside the boat mould during construction and the fibreglass laid up over it. Gelcoat will not cure in air (hence it cures under the layers of glass in construction), so my tip is to paint your coloured gelcoat over the repair then use an overhead projector clear transparency sheet and firmly press over the repair to give a smooth finish and exclude the air. Leave it over the gelcoat until it cures, peel of and sand with wet 240 through the grades to 1500+, finishing with a cutting paste. You may have a slightly proud repair, but it should bend in nicely and is not so important if you are using a keel guard to cover it. In your case, strength and integrity is vital, water ingress into the keel area is the kiss of death, particularly for foam filled stringer boats like Mariah's. If water ingresses into the repair, you could suffer from osmosis from the inside out and the outside in = bad news.
If you use an epoxy filler (West System is good), suggest that you prime and paint the surface with an antifouling as not may other paints are rated for below waterline use.
Depending on how deep your scratches are, gently try wet 360 and work your way up through the grades to wet 1500 then coarse cutting paste and you will get rid of the worst marks. If there are deeper ones, go easy, you do not want to go through the gelcoat layer as it will look even worse and you will loose the protection over the structure.
Good luck and post pics!
Looking for suggestions and recommendations
