Thanks to all for the helpful replies to my previous thread about flux. So I'm thinking I need to build a de-fluxing hot tub. A container large enough for a single frame, with a thermostatted heater from a hot-water heater to get the water to 125-150F, and maybe a small circulating pump to make the flux come off a little faster. Has anyone built or used something like this?
Mine is exactly like what you envision, except without the heater or the pump. In fact, if you really look closely, you'll note that it's awfully hard to see a difference between my set-up and a child's plastic pool with a garden hose in it.
Its been my experience that flux removal is more about duration in water than water temp. I've never met any flux that could stand up to overnight in the kiddie pool. Even the black stainless flux, no prob. There is a tendency to 'overtool' when just beginning. Tools are cool, but there is a certain elegance to doing more with less.
Actually, I've found that extremely hot water works very quickly. When I am in a hurry -- and the part is small enough to fit -- I dunk it in a bucket of near-boiling water for 5 minutes and then get on with it.
You're right though: overnight in the kiddie pool evaporates every trace of flux, even in 50 degree water. It's good enough for Richard Sachs...