1. Make sure to brush in one direction (going back and forth can push contamination back into the surface)
2. Absolutely get some acetone (Isopropyl alcohol works as well)
3. Try using low amperage to sweep over the contaminated area and let the cleaning action do its thing before doing a bead.
4. Don't start moving until the puddle is nice and shiny. You can poke a dab in there to help things along, but stay put until its clean and readily moves with your torch.
This is common and everyone deals with it. Hope this helps.