Re: does extra resin make repair weak?
Resin adds no real strength, it's the glass fibers that do the work and a resin rich layup will not be as strong as one at the correct ratio ( 33 to 40% glass, for hand laminating and 50 to 70% for infusion, or other types of closed molding).
If you were just trying to seal the wood and add a little strength, then a resin rich layup is sort of OK, but resin is brittle, so if it gets stressed or flexes, it may crack much easier.
3/4 oz mat is thin stuff, so you would need 4 + layers to build any strength and it's best to use roving along with it, roving is where you will get most of the strength.