To make a really clean casting depends on how good the mold is and the first beauty coat. The mold should be made of good silicone, molding latex, or urethane. Use a vacuum chamber if you have one. Make sure the silicone is poured from a high and small stream. Never pour right on top of the item, always in a corner.
Make a small batch of resin and brush or slosh a thin coat. The first coat is always the beauty coat. Use slow casting resin will help in making the coat bubble free. Use poly resin and you should not have any problems unless your mold is poo.
Well right now im using smoothcast 300Q series. This is a fast cating resin. Maybe thats the problem.
Because i dont have enough time (handling time is 30 sec.) i always just pour it in my silicone mold. Ans seen the fact that i dont have a vaccuun chamber doesnt make it easyer.
According to Smooth On, the 300Q doesn't need any degassing, same as the 320 that I use. However---you can never go wrong with degassing if you need to----in which case, you would need a resin with a longer pot life. I have never used a vacuum chamber, and have had pretty good luck with my castings. Every now and again I might get a bubble, in which case I mix a little dab of Bondo to fill it.
Clean Castings sometimes need clean molds.
Depending on molds and the corners inside the mold. If your molding process end up trapping bubbles while pouring silicone bubble cavity may form and when u cast something the resin may get trapped and thus a bubble mold! So for tiny details on sculpture its good to lightly brush on a thin layer and let it bubble out before doing the large batch. This is what I do for the Cavity pour system with a mother mold made first then pour in the silicone. its complicated but your silicone mold is precise and has keys to lock into the jacket mold.
For cleaner molds:
Pour in half to fill the mold with resin then tilt it around to wet the mold then pour the rest. Pour spout should be facing up always so the bubbles end up in the spout area and can be trimmed off later. Dont forget to use mold release. I use talcum powder to large clean sticky silicone molds after casting resin and brush off the excess flash from edges. I use another kind of spray mold release for smaller part molds. The resin I use cures in 2 mins in warm temperature. Never use any type of alcohol that may mix with your resin liquid. Chemicals will cause the resin to cure babdly and create bubbles.
I use Smooth-on 320 resin all the time and can get casts to come out bubble free with out using a vac chamber. I dust my molds with baby powder and tap out the excess before each pour, makes the part release easier and saves your molds. I don't use mold release when casting with resin only when doing silicone. Mold release leads to excess paint prep work
That's not entirely true, the company I used to work for made sensor head assemblies for pipeline inspection tools and all the resins were de-gassed in a pressure pot. Even the smallest air bubble would result in a part failure due to pressure. Now for prop and model making thats not really necessary as most of the resins we use have a short pot life and set up pretty fast.
I use talcum powder to make the mold less sticky, then i brush off dust and particles off the mold to make sure its clean. Silicone mold can be sticky and attract dirt easily. Also helps in demolding. I cast many bio's this way and have no problems.