I regularly buy various concentrations or "grades" in the 50's , 70's , 90's and 99%. The 90's I believe is the only percentage that won't cause rust, anything lower would.
If isopropyl alcohol doesn't work, you can switch to denatured alcohol. isopropyl or ethanol will dissolve the bulk of the crap I find on circuit boards. Ethanol (denatured alcohol) is what I regularly use when cleaning circuit boards after I repair them. I also don't find ethanols "sweeter" smell as annoying as the "medicine"-like isopropyl.