heh i made my very first cosplays from cardboard n duct tape... my family called me the duct tape king.. but thats another story...Kusangi is right though... you may want to check out the fibreglassing thread.. www.Bioweapons.com
has extensive tips (REALLY COOL GUYVER SUIT)
Ive recently embarked on that fibreglassing journey myself, with some shortcuts in between.
Priority mail envelopes are your friend! They're free(always good) and unlimited (sort of). If possible Id construct basic pieces from them and then covering the cardboard in Gaffers tape (or duct then another layer of masking, if Gaffers tape is out of your wallets reach)
Then put about 6 to 8 coats of Gesso, alternating directions with each coat. In other words on the first coat use vertical brush strokes, and on the second use horizontal ones...
Then in THEORY fibreglass right ontop of your cardboard wonder, Im not sure how many layes one or 2 should do it. Then a couple layers of Bondo (body filler) if you need to carve/ingrave details.
Finally WETsand your piece(make sure everything is COMPLETELY dry and then wet the surface to minimize fibreglass dust), and prime before painting!