LTF - From my experience the load U R trying to repair against is putting the current repair into compression - not its strong point...
Ideally U would do a mirror repair on the other side, but over a bigger area - pre-sand - make first patch biggest - 2 patch same size as current one and sand back to smooth 80/120 then say 180 finish with a 600 grit for a babies bottom finish (without the wrinkles

) - then mask up and spray a car acryllic finish if you want a mirror (used a can on my last fix - didn't bother with the spray gun)..
I like to pull the masking tape off after 80 plus minutes - but before set fully and once there is no obvious flow I turn the repaired surface facing down so gravity helps the finish. Just doing a minor work over - will post a pre sanding photo of surf board top deck dent infill & re-enforce (quickie needs to be ready 4 tomorrow so finish is lower down the priorities!!).
Hard choice, gardening or sanding this arvo??
WOW JackF how old is that board? My first UG circa 2002 fibreglass TT 157 looks like it came from the same family but bigger mould (bought from KP 2!!)

Hope ur fix works
Cheers
AP