Got this from Ziptied:
OK here it comes.
Allot of what drift girl said is good advise. Ill just highlight it and add a few more.
using 80 # grind away any fraid and jaged peices. also bevel the outside the part you see, down into the crack. Then be carefull and take away as much of the gell coat as you can with out going into the peice.
now on the back side grind it flat, All the jaged peices and the back side of the peices you need to creat adheision to the part hardend resin allone wont hold the patch.
now line up all your shit on a bench or what ever. If its the lower lip or something yes lay some wax paper down on a board and then set your bumper on it. If you have busted out peices that fit in where the bumper makes contact to the waxpaper lay them in.
HINT. this is going to get hot. from the chemical reaction so you might want to use some screws and hold that badboy in place. You dont need to go threw your part but allong the edge with fat headed screws just to keep things lined up.
There are many resins. cheep shit is probobly what they used in the first place so dont worry about blowing your wad on good resin. Average boat repair resin will do just fine.
Cut some strips out of mat some that are long like bandaids and some that will cover the area. I like to follow with some fine cloth so a peice or two of that if ya got it.
Using a 2 inch wide paint brush, brush on your mixed resin (FOLLOW THE DIRECTIONS IF ITS COLD OUT THROW A FEW EXTRA DROPS IN) paint the peice like your painting a house. Then lay your strips on. go oppisite of the crack with them like if the crack runs up and down run them left and right. If you have tight curves this is the place to but the strips togeather so you can make the radiuses.
Now brush on some more resin. while your doing this if you donot have a roller I take the brush and dab the resin into the mat to push out the air pockets. You need to get them out or your not building a stong repair.
After the strips are on and well coated and the air bubbles are out throw on your full sized patch. do the same with the aplication. If you have the cloth then throw one layer of that on and just dab it to soak up the execce.
This should dry in like a 45 mins to an hour if your impoatiant like me give it some heat with a heat lamp.
OK shits dried what to do next.
Get your 80# grinder back out you will notice that the fiberglass filled allot of the front so you need to grind it smooth. If you got hangies and sharpeis on the back side give it a polish with the grinder as well.
NOW to the front.
using (TIGER HAIR, KITTIE HAIR, LONG AND STRONG, FIBER THREE, CARBON FIBER PUTTY, OR WHAT EVER ELSE THEY HAVE) mix it up with its catalist and spread it on like bondo. This shit should kick fast like 20 to 30 mins. Then sand it with 36# cause its nasty hard. this should fill and make your surface real close to the pannles original shape. You will have pin holes. you might even have hard edges but this will fill with the
BONDO. yup finish it with that and do as you would any other bondo repair.
Dont wory about regell coating it. gell coat is not important in a repair. use a two part epoxy or 2 part primer to fill and sand it for paint.
Look in the spray can thread for painting instructions.
REMEMBER to grind further than you will be laying the epoxy and mat. dont get stupid with the grinder just remember to put a scratch into it to hold the new stuff. You can paint your resin out to there when your doing the rest and it will help bond it all into a rigid peice again