I bought a used Sigma 20mm f/1.4 DG Canon EF-mount lens which had its aperture and autofocus broken.
I fixed the autofocus simply by cleaning the switch but the aperture remains stiff. The aperture works in video mode if it goes from f/1.4 to f/16, but doesn't work in photo mode above f/3.5 because it gets stuck because of too much resistance. I tried cleaning the aperture with isopropyl alcohol but that didn't fix the problem.
What could I try to fix the aperture? Should I disassemble the mechanism and clean it comprehensively? I don't want to send it away for repair since that would cost more than what I paid for the lens. The rubber also reeks of cigarette smoke but that'll probably go away with time.