No problem although reading again my post, I could have written it a bit better.
I'm in the process of molding a lifecast and then do it in silicone.
When doing a glove mold (so the piece gets molding silicone/rubber on it entirely) you have no seam lines. Undercuts might become a problem if they are really pronounced. For the molding material (either u use silicone or polyurethane rubber) I would suggest something that can be brushed on otherwise it won't work. You don't have to degas your molding material because you apply the first coats of it with a brush. This way, you can capture all the detail and you have literally no air bubbles trapped.
Get some silicone pigments. Measure a good amount of silicone so that u don't end up with too less or too much. Premix a drop of silicone pigment (no matter what color) in the batch for the second coat. Don't worry, the color won't affect the color of the piece u are molding. When u color each with a pigment, you are able to see much better where u already applied the molding material and where not. When u do everything in the standard color (rubbers come in different standard colors, as f.e blue, white, green or orange) you might cover some areas too much or too less because you can't see exactly where u already applied material and how thick you've applied it.
When you have done 3 or 4 coats with your brush, capturing all the details, you can then build up the thickness of the rubber mold.
In order to do that, you can add a thickener to the silicone. This way, you can adjust the silicone to be like peanut butter or even thicker. Now you can get yourself a spatula and apply your thickened silicone to the piece (again, if you do more than one coat you can pigment the following coats in order to get e better overview). You just apply it like when you are frosting a cake.
Make sure when u applied your last coat of molding material, that the surface is done as smooth as you can so there will be no problematic locks between the mother mold (the rigid outer support shell) and your silicone mold. Keys are not necessary when doing this (keys are locks that hold the shape of the silicone mold when placed into the mother mold). You CAN add some keys here and there. Just pour some silicone into an ordinary small container (f.e a matchbox). Let it cure. Then remove the small block of silicone and cut off some square smaller blocks. These blocks you can glue on your mold once it's finished with a little bit of silicone (since these materials bond to themselfes).
After everything is cured, you can then go on and decide of what material the mother mold should be and if it's gonna be a two part or three part mold.
For this I think the two part mold works best. Just build a wall with water based or oil based clay around your model (along shoulders, neck, behind the ears, up the head sideways). And here you HAVE TO ADD keys! Otherwise the whole thing won't work.
Just use any round object you can find and press it into the clay wall at regular distances, not too much. When you apply your fiberglass matting, the half round dents in your wall transmit into the fiberglass. When the shell hardens, carefully remove your clay wall, apply release agent to the premade fiberglass shell flange (because otherwise the second flange will stick to the first and you will never get your piece out again). You can use mold wax to do so. Just try various materials avaiable for that. You can't get enough release agent.
Do the second shell (f.e back half of the mother mold), after all ready to handle drill some holes in the flange for screws to hold the jacked mold together.
Then, remove the two shells carefully, trying not to crack them. Pull the silicone away from the piece you've casted. Place it in one half of your mother mold, put everything together with the other half and voila you are ready to pour some silicone and slush cast a face...head..or whatever.
Note, that this is a very very primitive way to cast something but for a beginner this may help.
