Just remember what your dealing with
Fibreglass products are arse about face so your top coat (gelcoat) is the botton / start of the process
in a typical painted panel you have
Base substrate (metal plate)
E coat or similar (corrosion preventer)
Primer (one or two coats )
Top colour ( multiple coats )
Lacquer ( multiple coats )
to make a repair you rub through the lacquer, top coats & at worst back to primer
Firbreglasss is different you start with
Gelcoat/ pigment (top layer / visible surface)
Resin/ Mat (multiple layers) (substrate)
and thats it .
Rub through your gelcoat, and you will expose the glass fibres, get these wet, and you will get osmosis , so you nice repair may blister and pop some months later
any filler you use will not be the same colour as the Gelcoat , and you cant just whack a coat of paint over the gelcoat, as it may react & pickle
Ideally you need to remove all the loose and flaking gelcoat, and or rake any cracks back out to solid gelcoat
Think of a capital V thats what each line of the crack should look like
You then need to fill the repair with a blend of glass powder & gelcoat colour pigmented resin, this means you will get a near on ideal match to your original gelcoat.
you need to fill sightly overflush to allow for wet and drying back to level.
The you need to polish your repair to blend it in .
To do a simple repair you will need the following
Pigmented Resin & catylst
Fibreglass powder
Filling knife
Wet and dry paper
Fibreglass wax polish
you can buy ready mixed filler in a tin, but you still have the problem of you have to paint & colour match your gelcoat
You will need some glass fibre mat resin & catylst if your going to brace the inside with ally plate,
you will need to abrade the fibreglass otherwise the new resin will not stick eaither, and the first time you flex it it will pop the plate off the back
dont do it outdoors , the humidity is to high & the tamp too low at the moment, so it wil take ages to go off
if you do it indoors it will stink the place out