Those Ducts Are Dangerous

You would probably end up using a ton of silicone. Mastic on the collars would be a better choice.
Yeah, I've been cut many a time on sheet metal.

Ayuh,.... Tin cuts are like paper cuts, on steroids...

I use airtight fittings...comes with a gasket.

That fitting you have doesn't have good flow characteristics.

Yup, you gotta watch out for those maneaters.... they can dice you pretty good and sometimes you wont even know you cut yourself until later on.

Make sure to keep constant forward pressure while cutting. Slack off a bit and you'll end up with a "fish hook".

Work with sheet metal enough and your hands will toughen up or just learn how not to get cut, although I used to work with a guy who would constantly cut himself, even after years in the trade.

The worst I ever got cut was when I was trimming a collar off of a heavy gauge range hood transition fitting. I missed a huge fish hook and cut my finger down to the bone, 1" long, while rubbing the foil tape into the fittings. I even had my gloves in my pocket, lol. Went to the hospital and got the gash crazy glued shut because the doc said I would just tear the stitches out if I tried working with that hand.

Yep, its sharp and really hurts when you cut yourself with it.