I tried to find mastic before going with the tape and I could not find it at the local stores.

As far as using Duct Mastic on an operational system. You would want the system shut down for a while. But the stuff we use here in the factory is normally good to go in 24hrs. I'm sure the label on the tube/bucket is your best friend though.

Last year when I was going to do this to my house there wasn't any in stores either. I bought it from the modular factory where I work. But when I was reading this thread I looked on home depot site and it lists a tube and a 1/2gal. bucket of air duct sealant. I think the stores are coming around to the fact they have to start carrying this stuff.

Just a quick update on this. While I was insulating my pipes last week or so, I took another look at this to see how it was doing. Everything is still sticking and sealed fine.

That was some major improvements in covering up air leaks.

Keep an eye on the aluminum tape where it attaches to the wood. Mine held for about three years but eventually let go.

I was actually just in my basement and took a look at some of my taping done 8 years ago. Most of it is still good, but there are definitely area that are leaking large amounts of air again. Time to revisit things!

a quick and dirty reinforcement might be a couple of T50 staples across the wood/tape mating surface.