Copperfill and Bronzefill are REALLY brittle, not like glass but still a 5mm thick helmet will break when dropped on the floor.
Especially due to the copper and bronze particles the material is weak (the metal pulver doesnt improove physical properties) and the material is REALLY expensive because it’s priced by weight. Normal plastic has a density of around 1-1,1 gram / cm³.
Copperfill has 3,9 gram / cm³ meaning that your model weights more then 3 times more. With 3x the weight the cost triples too, i really dont think that you consider such an helmet out of a relatively instable, difficult to polish and 3x more expensive material.
When you’re sanding or anyhow machining plastics you should be aware of the temperature uprising on the surface.
Especially sanding causes alot of heat due to the high friction.
Plastics have different glass transition temperatures (called Tg), which specify the temperature spot at which the plastic gets soft and deformable (the deformation will stay once the pastic cooled).
Copperfill and alike are based on PLA, which has a Tg of 50-60C and let me tell you this temperature is reached within seconds while sanding, not to mention electric sanding tools.
ABS, Colorfabb HT, PMMA and PC have a Tg of around 100-130C which is much better and allows for rough and quick machining / sanding.
Colorfabb XT and Colorfabb NGEN have a Tg of 70 / 80C,which is not THAT good, but due to the physical properties of these 2 materials they dont build up heat that quickly. Besides ABS, PMMA and PC can be dimensionally instable (warping when printing large parts) so I would prefer Colorfabb any day (it’s my personal favourite filament supplier due to quality and price too).
Good luck and feel free to ask anything else that might concern you.
-Marius