NAGA prety much covered the reson/fiberglassing part... but if you are short on cash and need a cheep but fairly effective way to strengthen your armor, i'd use the HOT glue method...
the hot glue method is a way to layer the inside of your armor with hot glue. Here are some of the tings you will need...
A portable electric stove (might already have, otherwise, they are cheap)
Several Paint brushes with hairs, not foam brushes
Pie/coffee tin
Your armor piece
and hot glue (2 pakets is around enough for a helmet)
HOW TO USE...
plug in your electric stove and turn it on to around medum heat.
put your pie/coffie tin on the stove then add the hot glue. wait for it to melt.
using your paint brushes take some hot glue and apply it on the inside of your armor. Let it dry between layers.
after you have your desired thickness of glue, its ready to bondo/paint the outside!
SUPER HOT GLUE CAN AND WILL BURN SKIN!! WHERE LONG SLEEVES, PANTS, AND WORKSHOP GLOVES WHEN APPLYING GLUE!
p.s. if you are still using reson apply one layer to each side then glue.
Enjoy!