Problem with using acid-flux solder is that you have to carefully scrub the soldered parts off with soap and hot water after soldering so as to be *certain* that you've removed the last of the acid. Failure to do so will cause the acid-exposed parts to darken and corrode at the very least, and will also eat any paint off from underneath after it's been applied!
So while charging my motorcycle battery this morning it struck me: we use common baking soda to neutralize spilled battery acid, so why not use it after doing an acid-fluxed solder joint?
Tried it this afternoon after doing some long-put-off repairs on a brass loco, and can report that it works *just fine*!
I simply put a small amount of baking soda into an old 35mm film can, dipped a Q-Tip into some water, rolled the wet Q-Tip around in the soda for a second or two, and then spread the resulting thin paste liberally over the still-cooling solder joint.
Although invisible to the eye, the acid bubbled like mad for a few seconds when the soda hit it, and then subsided.
You still have to wash the soda-paste off of the joint in question after the bubbling stops, but there's no longer any doubt about whether or not the left-over acid has been completely neutralized!
The little film can of baking soda has now earned a place on the shelf beside all of my other soldering equipment.