SEL18200 Cleaning

So I'm trying to clean my sel18-200 because I have an accumulation of dust inside the front element due to frequent zooming (intake of air). There's a noticeable build up near the center of the element.

I've been trying to remove the front element to no avail. I even went about ordering a special rubber tool :

http://www.amazon.com/Repair-Silicone-Rubber-Retaining-Scratch/dp/B005YH7F16/ref=sr_1_2?ie=UTF8&qid=1330218888&sr=8-2

Unfortunately, attempting to unscrew the front retainer ring has been quite the issue. It won't seem to budge. I was curious if anyone else here has had any experience cleaning their 18200