Thank you all for the great advice and feedback.
I put the dents in the other night.
I sort of combined bfett81's and WPK's technique.
since i had already bent my armor :facepalm and didn't want it to loose its shape, We had to be very careful with how to put in the dents.
So, My dad got out his heat gun and heated the back of the armor, then we needed to figure out a way to put the dents in without ruining its shape.
So then we put cardboard(since we new it would indent with the sintra) behind the piece of armor that was to be dented and when the sintra was heated enough, we took various sized metal rounded objects and pushed it into the different places until we got the desired dent.
I think it turned out realy well.
And we didn't have to worry about too many toxic gasses.
now i just need to paint and weather it.