What is a simple and effective way to rinse? I just kept spraying, letting the excess to drip but it does not work.
I have an ultra sound cleaner that I use for bike chains, is it useful for PCBs? What solution? (I occasionally need to clean one off boards, no large quantities).
From the MG Chemical page...
Step 2 :
Rinsing the board You must ensure that you push the dissolved flux solids off of the board, before the flux remover evaporates and the solids re-deposit. If you are using flux remover in aerosol form, the easiest way to do this is to hold the board vertically and liberally apply more flux remover until you see the flux remover running off of the board. If you have dissolved the flux by submerging the board in liquid flux remover, just dip the board into the tray and swish the flux remover and you should be ok. If you have been using the particular tray to dissolve flux on a number of boards, dissolved flux may accumulate in the tray and dipping the board may actually deposit flux solids onto the board. In this case you will want to either set up a separate tray for rinsing, or have an aerosol can handy for rinsing. A particularly good option for rinsing dissolved flux off of a board is to use our 406B Super Wash. Super Wash comes in a large aerosol can, allowing for liberal use, and it dries very rapidly. Super Wash will rinse off un-evaporated flux remover as well as flux solids, and almost immediately after rinsing your board with it the board will be dry and ready for service. - See more at: http://www.mgchemicals.com/tech-support/instructional-guides/flux-removal/#sthash.m1tcpgq1.dpuf
That said, what you tried does work, but you may not have used enough of it (bold text).
They're in business to sell you chemicals of course, and they're not exactly inexpensive products when you find yourself going through a can on one or two boards, depending on size and how much crap has to be cleaned off.
IPA in the highest concentration you can get is a much less expensive solvent for removing flux, so something you might want to give a try and see how it works for you. Some ESD brushes will help too to break up heavy, stubborn deposits (cheap on eBay, aliexpress, or deal xtreme <dx.com>). Cheap dental picks/scrapers can be useful for this too.
You'd be able to use an ultrasonic cleaner, either with IPA or distilled water. Heated works even better, but don't leave the boards in there too long, as it can cause problems (heated or not). Experimentation would probably be the best way to go about it, no knowing anything about the unit you have.
Also make sure you're components can be immersed, as not all can be (check the datasheets).