I just now poured some castings, so since it's fresh on my mind, here's the exact steps I follow for casting in a basic open faced mold:
1) I pour some cornstarch baby powder into the mold. I push it around with a chip brush, making sure everything is covered well.
2) I pour out the excess powder, tapping the mold to help dislodge it. I also use the brush to remove some as well.
3) I blow into the mold to remove some more, flexing it around to make sure I get the excess out of the small crevices. There shouldn't be any piles left, just a very fine coat on the surfaces.
4) I measure out my resin into their respective cups. I was using Smoothcast 300, so I poured equal amounts into the small dixie cups.
*I was pressure casting these pieces, so skip steps 5-9 if you're not using a pressure pot and ignore the references to psi and the pressure pot in steps 10-11*
5) I double check that I have my mold positioned correctly in the pot, make sure I have enough pressure in my compressor, etc. Basically just giving everything a once over since the 300 is going to kick quickly and I don't have time to do anything else that I might have forgotten.
6) I mix the resin together, trying to not create a bunch of extra air bubbles in it during the process, but making sure it is thoroughly mixed. I'm using a small amount, so it doesn't take but maybe 20 second of stirring.
7) I pour the resin into the mold, making sure it fills all the cavities. If it doesn't I use my stirring stick to push it around and get it in there. (I'm using a gang mold)
8) I close up my pot, making sure that the lid is tight. Even if I take too long and get a bad pour, it's still much cheaper to trash a bit of resin than the alternative of not getting the lid tight and having a large accident.
9) I open my valve to pressurize the pot. I found that 40psi is adequate for these pieces, so I let it go to 42psi, just to be sure (my safety valve is 55psi, so I've got plenty of wiggle room)
10) I close my valve at 42psi and leave it sitting for about 10-15 minutes.
11) After that time has expired, I use the same valve to relieve the pressure and make sure all pressure is off the pot. I leave the valve open while I'm taking the lid off just to be sure.
12) I remove the mold from the pot and remove the cast parts from the mold.
13) If need be, I sand the flashing off the parts and they're now ready for finishing.
As for making a mold. If I'm making an open face mold, I usually place my parts on a flat bed of clay (super sculpey) or will glue the parts on a sheet of plexiglass (if I won't need the original parts again). I surround it with whatever is simplest depending on size and shape (legos, wood box, pvc pipe, styrene wrapped around, etc. When on a bed of clay, I just push it down into there, if on the plexi, I'll use hot glue to seal it down. I normally only mold non-porous originals, so at this point I'm ready to pour the rubber. I don't use mold release for open face molds.
When doing a two part mold, that's the only time I'll use mold release. Not for the part itself, but as a covering for the cured silicone, so the the two halves don't stick together. I like 3-4 coats of mold release, with adequate drying time in between. I've never had the silicone stick one bit when using that number of coats. Even if I miss a spot with once coat, the next usually finds it.
For parts that come out of a mold that has been exposed to mold release, I'll wash them off with normal soap and water, then give them two scrubbings with Castrol Super Clean, then one final wash with soap and water, followed by a good rinse and plenty of drying time.
As for your question about what type of paint/primer to use. It doesn't matter that much if you're using a tin cure silicone. Just get the surface finished the way you want the castings (very smooth, slight texture, etc.) To an extent, you can determine the sheen of the part with just the surface of your master (if you're using tinted resin and won't be painting). For example spraying the master with a coat of flat painting and not sanding it will give you a matte finish on your cast. Extremely smooth surface (glass smooth) will give you more of a gloss finish.
