How to clean solder flux paste
What percentage isopropyl alcohol?  70% is not enough. Even 90% is a bit low. I use >99.9% which can sometimes be found in drug stores. Sometimes even that feels low. Ethanol works maybe 50-100% better and smells better too but costs 4x. You don't really need ethanol as long as you can scrub though. 

It takes rubbing with FRESH areas of a paper towel. Flux saturated yellow/brown areas of paper towel won't rub away flux anymore. Move around to clean areas of the towel. You can rub a thick, sticky flux spot and it will look like nothing has changed, but flip the paper towel over and see its colour. If it is bright yellow  or dark brown then you did remove some residue but it was so thick you couldn't tell. So move to a fresh piece of towel. It can take 5 passes or more for really thick residue. Just keep flipping the towel over, looking at it, and moving to a clean spot soaked in alcohol.

Brushes help too but mainly as a substitute for your fingers to press the paper towel down into nooks and crannies. I never found a brush (stiff like a toothbrush or even stiffer) alone to be very effective unless submerged since it can scrub off residue but can't pick it up. Helps keep the brush clean too if you use it through a towel, especially if you are using a $30 ESD brush.