I was reading that the reason why some solder flux tends to spit is due to 2 reasons. The first being the pressure built up from being within the core which is why some people use solder feeders ( I actually just learned this a few minutes ago today!) The feeder cuts a "v" shape groove which helps to release the pressure build up but as a caveat only use what you need because solder cannot be stored for long periods of time since the air will contaminate the flux. My guess causing it to evap the alcohol content as well cause the metal from the solder to rust. Reason #2 being that the activity of the flux being strong. So since you mentioned you use kester which is great stuff I'm going to take a guess you are using the "44" line which is RA flux (Rosin Activated) I'm sure you already know this but writing it for the rookies or people who may not know, it is the strongest type of flux. It is said that when the heat reacts with the flux it causes it to spit since it is aggressive, and I can co-sign because it always happens to me while using the 63/37 44 configuration. Excellent video always look forward to Nes mods and it would be cool to see you partnered up with bonzo he's also a cool guy. If any soldering pros out there that see this info and would like to correct it if anything is wrong or would like to add more, feel free. I'm all for learning more!

So, in theory, kester's RMA 285, since it's midly activated it should do it less if not at all. I'm going to put it to the test soon to confirm, although I don't have a feeder, but based on the information I have a few different flux types of kester's solder spools to do a mini shootout.