I agree w/ Carpathia, the combo of both rondo & fiberglass really doesn't make things too heavy in the grand scheme of things. I was working on my 'Rookie' ODST helm this afternoon and started w/ fiberglass cloth along the top of the helmet then switched to rondo for the sides where there's more fine details.
I suppose if you really get down to it, from a standpoint of realism, if these parts were made of the real materials our suits would weigh half a ton (Spartans) and probably something like 50-100lbs for ODSTs (not counting weapons, ammo, equipment, dedicated comms, etc) But thankfully we don't have to be super real to look awesome
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It's all about balance: strength vs. appearance...you just have to work with your chosen medium and know how much you need to get the job done, and how much you can add for rigidity without detracting from how it looks. Tons of rondo will be strong/stiff, so will fiberglass cloth/resin, but using a whole bunch of just one of them presents its own issues.
TL;DR fiberglass the easy to reach spots, Rondo the tough spots, let it all cure, then see what you can reinforce ON TOP of (rondo).