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
Scissors
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.

Some imagination.

Step 2: The Base for All the Helmets

Begin by measuring the head of the wearer. Do this from just above the eyebrow, and over the top of the ears. Add 15mm or 1/2" to the measurement and then cut a strip of card about 50mm or 2" across long enough to form a circle of the correct size. I add about an inch of material to the strip for joining purposes. My measurement was 23" so I cut the strip 24" long.
Glue the circle together using general purpose glue. Allow to set thoroughly.

Test fit the circle to the wearer. It should not be tight, or even snug. If it is start over, it's important to get this first part right. The fit should be loose enough that you can get your finger between the card circle and your head.

Next, cut two strips of card long enough to reach from the base of the circle, over your head to the base of the other side of the circle, mine were around 300mm or 12" long. This allowed a gap above my head but that's not a problem. The length of these strips will determine the height and ultimately the shape of the dome of the helmet. The width of the strips should be between 25mm (1") and 50mm (2"). The thinner the strips, the more filler pieces will be needed and the smoother the basic dome will be. But that requires a lot more work at this stage. It will save work later, but my helmet did not require a smooth dome.
Glue the two strips together to form a cross, this will act as the main support for the dome of the helmet. Allow to dry thoroughly.

Now for the first set of decisions. You can fit and glue the cross INSIDE the circle we made earlier. This will leave a crest ridge round the outside of your helmet, this what I wanted for this design. However, you can glue the cross to the OUTSIDE of the circle which will form a dome suitable for a Spartan or Greek type of helmet instead.
Glue or tape the cross in to place.

Next we need to make filler pieces to complete the basic dome. I've used 4 pieces of thin card for this but you can use 8 or even 16 pieces forming a much smoother basic dome. It just requires a lot more work now, and that means less later. I cut the pieces using trial and error. They were then taped into place using masking tape. All the joints need to be sealed with tape.

Test fit again, you should still be able to get a finger between the circle and your head and the very top of the dome should not be in contact with your head. A small gap is fine, but there should be a gap.

This basic structure is the basis for all my helmets. From this point on, you will have to decide what kind of helmet you want to end up with. Get hold of some reference materials to work out how to proceed.

Step 3: Making the Front Plate.

My helmet has an integral front plate and side jowls. How you approach the next step will depend upon your final design.
I used my references to draw out a basic template, I glued this on to the back of a piece of card with spray mount, drew in the missing sections, then cut it out using a scalpel.

Bend the plate to shape and then test tack it to your dome using masking tape. Try the helmet on, re-position the front plate as required. If it is too close to your face add some card packing between the front plate and the dome until it is a comfortable fit. Then secure it properly with masking tape.

I have marked the front of my helmet with some very basic lines to show where various joints and ridges will need to go.
I based my helmet loosely on the Witch King of Angmar from LOTR. As you can see, this is not going to be an exact copy of the original, but I'm not bothered about that, I just want something similar. If you want to make an exact copy go right ahead and alter the design to suit.

I then added a couple of thin card ridges to my basic plate to act as a guide when final sculpting takes place.

Step 4: First Strengthening and Adding the Jowl Plates

I decided that it was now time to start strengthening the helmet.

I mix up small amounts of the liquid resin in a plastic cup, then brush a good single layer all around the inside of the helmet. I use small amounts at a time because this resin is only workable for about 5 or 6 minutes before it begins to gel.

Set it aside to dry. Once the inside has cured give the outside a good single coat. Once it has all hardened off (about an hour), then sand back any major drips with the Dremel. Check to see if there are any sharp edges on the inside and sand them back as well.

Repeat this process with a second layer of resin both inside and out and then leave it overnight. By the morning you will find that the helmet is MUCH stronger than it was! Again do a test fitting to make sure it all still fits, after this point you will have massive problems trying to adjust the fit!

Next I marked out and cut the two side jowl sections. These were then taped into place and trimmed over the ears to fit. I kept test fitting as I went until the fit was comfortable. Then I gave the side panels an inside and outside coat of resin and left it to cure.

The structure should have a 'hard' feel, but still be slightly flexible. I decided that the two front 'jaw' sections of my mask were a little too flexible. So I added another coat of resin to these and put some glass fibre tissue into the wet resin and then brushed over the top.
Once all of that had set, I sanded back the loose tissue and stay resin. the jaws were then much stronger.

If you want to add a neck flare at the back, do that now and then resin coat that as well. My design won't need one.

Step 5: Texturing

Once we have a solid basic helmet it is time to make any ridges and surface textures. For a smooth domed helmet you will need to cover the entire dome in the filler paste. My helmet won't need that, but I wanted to form ridges in the surfaces and give the helmet a rough forged look.

Mix the paste in small amounts. Any bigger than golf ball size and you will end up trying to add paste that has already started to go off. The P38 is supplied with some small plastic smoothers. I use these to mix each batch and apply it. This was done fairly roughly, allowed to dry and then sanded back.

Continue to add more paste, allow to dry and sand back. Repeat this process until all the outer surfaces are covered and finished.

For a smooth finish, once the basic shapes are made, you can sand with ever finer grades of wet and dry paper.

Once you have completed this stage you can add any additional bits such as rivets, filigree work or spikes and horns. You can also add the holder for a crest if you have covered the dome.

Step 6: Painting and Finishing

I tend to use acrylic paints and inks to finish my constructions. I use brush, sponge and airbrush to apply the paints. It is possible to do the whole thing using a brush, I just find it quicker and more subtle to use the airbrush now and again.

There are various ways to prepare a 'metallic' finish, and depending on the final colour you want to achieve, all of them require layering to get some sort of realistic look to the final mask.

For Iron, Steel and Bronze start with a matt black base coat. I often use car body spray paint available in cans. For Gold, Brass or Polished metal, you need to base coat in a slightly lighter colour, such as a brown for gold and brass or a gunmetal for polished steel.

Once the base coat has dried, start adding ever lighter colours. I used a sponge, took my time and delicately added lighter colours to the upper surfaces. Then I gently airbrushed a little more black on the lower surfaces to give the impression of shadows. this gives the entire thing a more 3 dimensional appearance.

Finally I dry brushed the entire thing in a light silver (I used Citadel ' Mithril Silver' from Games Workshop). This finally brings out the highlights, and completes the paint section.

The dome of my helmet will be covered in black fabric to finish.
I like the skull sort of appearance of the mask, but I might decide to add some black material to the mouth area and possibly some black mesh for the eyes to give the illusion of the faceless warrior.