[Ant]

You need to grind a wedge on the front side going back approx 2", the edge needs to be sharp as no butt glueing is recommended as there is no strength or surface area.
The wedge or scarf in boats is 1/2" per ply, if I am bonding a new flare on my cobra I do about a 3" taper scarf going right down through the gelcoat, I try and avoid bonding directly to gelcoat as sometimes it can be really thick and there is no strength in coloured resin, so I need to see glass fibres!

Polyester resin relies on the abraded area and fibres in it for strength, it is not a great glue like epoxy which is used for structural areas etc. even with epoxy I would use the same scarfing area technique, my front guard flares have been remade in epoxy due to a damage repair and you cant lay polyester over epoxy.