I wanted to make a mask / helmet for a fancy dress ball. The method used here could construct any rough and battle damaged type of helmet, from ancient Sparta and Greece to Roman or even Medieval or LOTR.
A smooth surface can be achieved but it will require a lot of elbow grease to sand back. A rough finish is quicker and easier.
The basic system I use for almost all my solid props and masks is well known to the Halo boys. It's basic structure is card, strengthened with car repair resin and then shaped with filler.
Final finishes, in my case are generally acrylic paints.
Step 1: Materials
In my other 'ibles there seems to be some confusion regarding the two polyurethane resins I use.
The first is a liquid car body repair resin designed for bonding Fibreglass / Glass Fibre matting. It comes as a straw coloured liquid to which a catalyst hardener is added. In the UK I use David's Fastglass resin because it is readily available (from Halfords) and it is very forgiving of the user when it comes to the amount of hardener added. It's not massively critical. When mixed it takes on a red colour allowing you to see an even mix. It can be poured, cast, brushed on etc.
The second is a paste or putty. Again mostly used for filling jobs on car bodies. Here in the UK I use Isopon P38, again readily available (at Halfords), again you mix in a catalyst hardener which allows you to see a colour change and an even mix. Again not too critical of amounts.
In the US a similar product is manufactured by Bondo. Check out the auto repair dealers in your own country for similar products. Use all normal safety precautions, these products are general pretty noxious and smell terrible - use in a ventilated area. I tend to wear latex gloves when handling them too.
Other materials required are:
Card - various thickness and types. I use scrap for all my projects.
Tape - masking tape is best, plastic types tend to melt
Glue - Regular general purpose glue like Bostik, and possibly a hot melt gun.
Some very sharp craft knives - Swann and Morton scalpels are my choice, but X-Acto or similar will do
Steel ruler comes in handy, and a cutting mat if you have one.
Some glass fibre matting or tissue.
Rotary tool (like a Dremel) with various tools and bits for sanding and shaping.