If I were to buy a bodykit or lip kit and wanted to add more fiberglass to the backside so it was more sturdy, would I just buy fiberglass resin in a tub and apply it with a brush or do I have to do something else? Do I have to finish it with something or can I leave it as is?

 

That's pretty much all there is to it (other than adding the catalyst to the resin.) Just make sure you clean up any spills ASAFP!

 

No. The resin is the weak link. The strength comes from the glass. You need to add glass, either chopped or (easier to me) as woven mat. Clean and sand the surface, then add wet glass. The resin holds the glass together once it cures.

 

oh...right...I missed that (crucial) part in OP's question

 

Mix the hardener into the resin then brush it onto the sanded part. Apply the matting then brush some more of the mixed resin over it, making sure you get all of the air out. When laying the matting start with the smallest piece and work your way up to the largest for proper adhesion.