I have recently finished soldering all my components to a PCB. I then used spray Isopropyl Alcohol (99.9% pure anhydrous) and sprayed the board multiple times and then used an acid brush to try to get all the flux off the board. But I have found no matter how much I do this there remains a thin film of flux on the board (you can tell the board is slightly sticky). Is there a different chemical I should be using or a different technique? Thanks in advance.