Will 20 psi get out air bubbles when resin casting? I’ve been trying to figure out how much psi I need to get air bubbles out of resin when casting. Does anyone have experience with this? Can anyone tell me what amount of pressure will get rid of air bubbles?
The amount of vacuum pressure required to draw out all the bubbles is likely dependent on the specific weight and chemical composition of the resin you are using. That should be in your resin's spec. sheet or technical data available from the manufacturer. 

Or you could just eyeball it like blacksmith37 says.