I doubt that paint contamination made it worse. We weld aircraft without scraping off the paint and it's fine for hundred of thousands of cycles, we just monitor it. Worst case is it isn't as good as it could have been. If it were me I'd stop drill the crack like someone suggested before even now after the weld job in case the crack wasn't not completely "caught" in the weld job. Doesn't need to be a big drill point, just a tiny one with a larger radius than a crack, it will reduce the stress concentrations drastically. And if it isn't cracked a small drill hole inside an otherwise "whole" bar isn't going to cause catastrophic failure. Good things about Aluminum/Steel: There is a lot of time between a defect forming and failure. If you make a habit of checking that weld before every ride you will catch anything amiss long before it becomes an issue.
Otherwise ride on, ride on.