1/1 scale busts! My fave!
Here's my preffered method:
Molding- RTV all the way. Three times the price of latex, but with two important advantages: (1) Latex shrinks as it dries, and the bigger the sculpt, the more pronounced the effects of the shrinking. This can and does cause features to come out distorted in the final casting. Moisture can also be reintroduced to the rubber through humidity or exposure to "humid" materials (like plaster), this moisture can then cause surface flaws in the casting (and you won't always know it's even present untill you demold) , as well as compounding/advancing the distortion issues mentioned above. (2) Latex degrades harshly with time and use. If the fist two castings are unsuitable for some reason, the third will suffer the damages done to the mold by those before it.
For molding a 1/1 bust in RTV, you actually don't need much rubber, as you'll want to use a different mold buiding method than is common to kit part casting. Believe it or not, a one quart kit is sufficiant to mold a human head sized sculpt. Mix in three batches, first should be small, and painted on as a lay-up coat. Do NOT make a clay divider for the seam at this point, just cover the whole sculpt evenly.
Mix a little slightly rubber than you need for the lay-up itself. Before you start, roll out a flat sheet of clay, and stamp a bunch of depressions in it using a large hex nut. Fill these stamped depressions with the excess rubber from the lay-up. More on those in a moment.
Once the lay-up coat has set, then you start with the clay wall. Instead of a full two-piece mold, I much preffer a single piece mold with a half seam running up the spine to the crown of the head. This saves time and effort, Both in the moldmaking, and the clean-up of the final casting. Making a full two piece mold is not only totally unnecessary for this type of mold, but actually a liability in that it makes acheaving a clean seam alignment during casting slightly trickier.
Mix the second batch of rubber, and use it to laminate pieces of spandex cloth into the second layer. The second layer should be done it two stages, first an all around wrap/coat covering all but the rear face of the clay seam devider. The spandex will reinforce the rubber while remaining elastic & flexable, allowing you to make a "glove mold" like this without worrying about the relatively thin rubber layer tearing during demolding. While the application is still setting, take those hex stamp bits of rubber you made during the lay-up, and slap 'em all over the surface like pepperoni on a pizza. When the rubber is cured, these will become posative keys for the plaster mother mold.
The third stage is simply to provide the other half of the seam after the first stage has set and the clay devider removed. Use the spandex 'n rubber combo here too. Spread seperator on the mating surface of the seam before applying the second half of the seam, but no where else.
The mother mold can simply be traditional two-piece plaster and burlap affair. Seal the inside of the mother mold with shellack or clear laquer.
Once the whole mold is finished and cured, remove the rubber from the sculpt by carefully tearing or cutting the thin fascia of the lay-up coat inside the bottom of the seam. The reason for doing the lay-up before the actual clayed seam is because this easaly and reliably gets you an immaculate seam on the final casting, while reducing fuss and risk to your master during mold construction.
Casting: I like to use finishing epoxy resin for the "skin", with an expanding urathane foam core for reduced weight and better economy. The finishing epoxy I get from Home Depot, it's safer and easeir to work with than the stuff one gets from a plastics or marine supplier. It mixes easaly in a 1/1 ratio, and can be tinted with any oil based paint (I like stained glass enamels, as they can be mixed to make a very lifelike translucent flesh tone in the resin), it is also less viscous than glue epoxies or the EnviroTex finishing epoxy found at craft stores. It is also MUCH more durable than most common polurathane resins. the expanding foam I get from my local hobby shop, it's sold for filling RC aircraft bodies, as well as for some railroad scenery applications. WAY better handling than the aerosol "Great Stuff" foam from Home Depot, for only maybe two bucks more.
IMO, latex sucks for this sort of thing. It's cheap, and easy to use, but it ages badly (and quickly), and is severely lacking in the lifelike realism department in several ways. Then again, I set high standards for myself. For many latex might be in the "close enough is good enough" range, but for me, it's results are flat unnacceptable.
First roll out thin sheets of clay, and layer them onto the inside surfaces of the rubber mold, shooting for an even thickness of about 1cm. Do this while the rubber mold is secure inside the mother mold (it helps to vasaline, or somethng similarly sticky and non-permanent to tack the rubber to the plaster). Once you've got that set, dust the clay with talk, and dab or spray with thinned PVA/white glue, and pour the foam. The PVA will create a skin for the urathane foam, to keep it from gluing itself to the clay, and the talc in turn will help the PVA seperate from the clay (neither will do by itself).
Forgot to mention: it's a good idea while prepping the master for moldmaking to create a blank "collar" at the base of the sculpt with sculpted-in keys for the foam. The resulting negative keys in the mold rubber will be left uncovered by the clay during the above process, so that the cured foam core will be secure while "floating" in the mold cavity during the resin stage.
When the foam is cured, empty the mold, and peel off the clay off the foam. Clean the mold and the foam core, don't worry about removing the PVA skin on the foam: it's supposed to be permanent, just as long as the clay's all gone. Drill a large hole (about 1.5-2cm) through the center of the foam core, from the base to the crown of the head. This will become the gate for pouring the resin. Resecure the rubber mold in the plaster mother mold with the foam core set into it's keys at the collar, and you're ready for the resin.
The resin most be mixed carefully and poured carefully to avoid airbubbles. It is a good idea to poke or drill vent holes from the low points on the sculpture's surface throught the foam core to the "top" (actually the underside of the neck/shoulder area, as the mold will natually be inverted during pouring). Pout the resin through the gate in the foam core, tipping and rotating the mold as you go, onlill it begins to push out of the vents at the top. Set the mold on a stable surface, and tap it's surfaces to dislodge any large air bubbles. Epoxy resin is kinda viscous, so this can be tricky. Most I've tried are like honey, although the Home depot finishing resin I currenty like has a near liquid (like thick cream) consitancy, witch is further improved by the enamels used to tint it, so it's not so bad. In addition, you want to design your molding/casting procedure so that you never have epoxy resin in more than about a 1-2cm thickness anywhere. I've never had any problems with heat buildup using the 24-48 hour slower setting stuff, but I have heard horror stories from others about what can happen if one did, so better safe than sorry.
The finishing resin takes about 48 hours to cure, but can be demolded earlier. It is actually advantagous to work with the casting before it has fully cured, as its consistancy at this stage allows for better paint adhesion (no primer, or you ruin the lifelike flesh effect), as well as hair punching.
This is just my way. I'm not a professional though (and I've only done this three times myself), others undoubtably know better ways. Looking back, it seems a little tricky in places, and definately more expensive, but I feel it's worth it for that extra dimension of "Madame Tussauds" type realism.
Hope this helps
*edit* Wow! Sorry for the hugeness of the above post.
"Chaos is found in greatest abundance wherever order is being sought. It always defeats order, because it is better organized."
-Ly Tin Wheedle
). I've never made a mold before, so I don't know if I should vacuform this or do the resin thing? I'd like it to be hollow so I can put in some acrylic eyes. Or would it be easier to just make a plaster cast? 
