Stock bumper? I'm not sure about doing repairs, I havent even started my first project yet, only been in the research phase. I read the procedure in a book on repairing composites. This is mosy likely not word for word or a good "drawing" but I will try.
If you have a crack clean thru a part start by using a blade or something else to v-groove the top of the crack. Top being the outside of your bumper the part you can see. Bottom being the inside.
Crude drawing to follow.
Before:
Top
||
Bottom
After:
Top
\/
Bottom
Now tape the bumper together on the top to hold the crack in place the best you can. Turn the bumper over and rough up the urethane a bit with some sandpaper, this may not be neccesary, so the fiberglass resin has somethign to stick to. Apply a thin layer of resin to the crack area, try to not let too much resin seap through the crack to the front side. To that thin layer apply some fiberglass mat and soak that with some resin. Apply a few more layers of resin and mat to build up strength around the crack area. Once that has fully cured and you feel it's strong enough it's time to move back to the front.
Remember that V groove you made? You will now fill the groove with body filler. Bondo would prolly work fine, but I suggest bondo-hair, which is body filler with fiberglass strands in it. Once the groove is filled it's time to sand smooth and prep for painting.
This may all be way above and beyond what you want to do, but that is how I understood it from reading. Try to google for "repairing plastic body parts" you may come up with a tutorial that has pictures. I have seen things online for repairing snowmobile and jetski bodies, it is the same process.
