wattsabundant
Chief Journeyman
Sometimes it's best to take a small jewelers flat tip screwdriver and start carefully pushing and scraping that old flux off. It usually shears off in good sized pieces because it's so old and hard. Then clean normally. Just wear eye protection. Those flux chip can be sharp.
"Just wear eye protection." Somewhere in the '80s i got a solder blob in my eye. I recovered ok. Since then I have safety glasses on for most everything I do. Weed eater, chain saw, it's safety glasses and face shield. You don't always get a second chance.