Work from the back of the join, rough up both surfaces with some rough (240 grit) paper then support both peices on a home made jig (i.e. clamp them to something stable and in the correct position) then thigs start to get messy....
mix the hardner and glass resin in a pot, spread some over the joint area (much larger than you need then cut the fibreglass sheets to size, lay them on the joint and using a stiff brush push the fibreglass matt into the resin you already brushed on, then brush more resin and apply more matting making sure you dont trap any air under the matting, smaller bits of matt are easier to work with.
When you have about 3 layers start to use slightly larger pieces to cover the whole work area apply keep going until you have about 6-8 layers in total and let it dry and harden for the specified time (the last peice you put on should be 1 huge flat sheet that you can rub flat without disturbing any smaller bits of matt underneath !
When this is dry and hard flip the section over and run resin into the joint gap on top and let it harden and smooth it off, Or use filler (bondo) in the gap.
Some people like to incorporate a metal strip across the join which gets covered in the fibreglass, this adds strength and needs to be in place at the begining of the process.
Fiberglass gets pretty hot when you mix the resin so wear gloves as it can really irritate the skin.
Hope this helps.
