Question is, should I then mix up some resin/hardener and fill any gaps and let that dry, before applying the fibreglass tape?
I would do this as a 'wet on wet' repair....
Roughen the repair area and wipe clean with some acetone.
Mix some epoxy thickened with colloidal silica (mix a small batch of epoxy as directed say 50ml, easy maths with the 5:1 ratio for West Epoxy) then add approx the same volume again of silica into the cup. Mix thoroughly and wear a dust mask, silica is best kept out of your lungs! I would then use a spreader to push the thickened epoxy over and around the repair area with the specific aim of working as much as possible into the crack. You won't use all the epoxy you have mixed. Do this on the inside and outside of the hull.
Then apply some tape to the crack on both sides. I would use a lightweight tape for this job to reduce it's cosmetic impact, say 100-150g/m 50mm wide tape. If you pre-cut the tape beforehand it will save getting into a sticky mess with scissors now.
Mix a fresh similar sized batch of epoxy, this time unthickened. Using a 1" disposable brush (it helps to cut off half the length of the bristles to stiffen the brush) stipple the unthickened epoxy into the cloth over the repair area. The tape should go clear as it is saturated with the resin. Try to get a good neat wet out of the tape without flooding the area with masses of resin. Ideally you want a nice smooth surface just filling the weave of the cloth. You have plenty of working time to do this right on a small repair and the epoxy will flow and level itself a little.
I'm guessing your boat is painted. Before overpainting the epoxy you will need to remove the 'amine blush' this is a waxy surface layer created as the epoxy cures. This can be removed by lightly sanding with some wet and dry or with soapy water and a green pot scouring pad. Use wet and dry to try and get as fair (smooth in line with the curves of the boat) a finish as possible, but go slow and be very careful it is so easy to sand too much and make things worse!
Epoxy and silica etc can be nasty stuff. Please wear eye protection, disposable gloves throughout and a dust mask when the silica is unmixed.
Scoops