Over on the synth-builders forums I frequent the recommendations are always 331 for washable parts and 245 for the remainder. I’m very interested to hear that 245 is not ideal for hand soldering. Hmmm...
(I’ve always used multicore to date, no issues, but was wondering about trying out some Kester)
I recently switched over to kester 331 63/37 and I love it, the boards gets super clean with simply some tap and di water. My ocd can't handle flux residue and spatter all over the place so this stuff is a life saver, no more trying to scrub of no-clean flux with heaps of ipa.
Use multicore 60/40 crystal 502 since I have a huge roll for mechanical and non-washable parts.
There's your problem... "no clean" flux is not meant to be cleaned, and certainly not with IPA. If it really needs to be cleaned you need a Kester-specified saponifier or cleaning agent, and you need to clean extremely well or you just spread the residue all over board. IPA will not do the job and will leave all sorts of nasty residue.
Most any Rosin-based flux will clean quite nicely with IPA. It's a little trickier if it burns/chars but it is still quite easy. If the IPA is
too pure (there is such a thing) then you might need to add just a little water to the initial scrub to help dissolve stuff that IPA itself can't do. Final rinse with the purest IPA you have will help remove almost all traces of water.
You will find that Kester 44 (RA) or 285 (RMA) clean up easily and solder well. And they smell not too bad as well, certainly better than most no-clean or water soluble fluxes.
I have some Alpha Cleanline 7000, which has some kind of mystical unicorn "no clean water soluble" flux. Apparently perfectly safe to leave on the board, but also easily cleaned with water. I don't use it because it smells nasty, which is probably due to all the active parts of the flux being designed to vaporize during soldering, which is how they achieve the no-clean part.
My recommendation is Kester 44 in either 63/37 or if you can afford it 62/36/2 (2% silver). The silver bearing solder is worth it, it melts at a lower temperature and the joints will be smoother and shinier than any you've ever made. It's also stronger, and it won't leach silver away from silver-plated parts. The 44 flux is the best I've ever used, making solder flow like melted butter, and it tackles tough oxidized stuff where lesser fluxes fail. It cleans easily with IPA but can be left behind if you don't care about looks.