A No-clean flux is designed to encapsulate, and render effectively non-mobile
(read here as uncleanable) the residues left behind.
You can clean it off, it is simply that it will not be easy, and may involve
finding chemistry that is not currently available.
Have you given any thought to using a "cleanable" flux? This would be an
awful lot easier.
Rudy Sedlak
RD Chemical Company