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| p.1 #5 · Sigma's Sticky Zen And 500/7.2 APO Examples |
I had the 70-200, not with this finish but the typical textured sigma material: my experience was that the finish actually flaked off. What I did, was buy a can of Fixatif spray from a local art store, mask the front glass and mount, and spray in layers the remainder of the lens. The spray is non acidic, non corrosive (neutral), clear, UV resistant and thin enough not to interfere with the operation. I sprayed everything, including the grip, aperture window, mount. It worked for the remainder of the year and a bit I had the lens.
You may want to try a small part of the lens to se if it actuall stops the breakdown.
What you are showing, to me, looks like UV degredation; not sure if you can stop it, my suggestion here is for sealing it.