Depends on the dent, hole, cut..goulge? I have had great success with four products: FIRST..clean the area of dirt and particles.
Gorilla glue, spray a little water mist on the part you want filled..place this glue in the part where the chunk came out...it will foam up and you can work it with a popsicle stick or balsa stick...work it so its taking the same shape. Dont get concerned if the glues expands beyond the hole. Once dry you can cut access away and then sand to a nice smooth surface. This very light stuff.
Plummers foam in a can: Spray the foam in the area that was damaged, again this will expand fast...be easy on the foam. let it dry completely, take a sharp razor and trim off the excess, then sand and paint. This is good for filling large gaps, the bad thing is, once you use it..the can is no good for saving. Fill as many dents and holes as possible. http://www.touch-n-foam.com/maxfill.php
If your near ocean..you can go to the surf shop and get UV filler. This product is for filling dents and "chunks" on surfboards, I have used this on planes as well. Follow the directions..you must use this indoors..it cures very fast in the sun in less than a minute. Sand it..paint it...if its not exactly right apply more indoors, shape it..and run outdoors in the sun again...boom its hard as fiberglass. http://www.amazon.com/Ding-All-Fille.../dp/B003GX80IW
Bondo (for auto repairs)