Are you trying to seal connections between your duct work? If so, then consider using duct mastic. This is what professionals use.

Here is a link with a video on how to use it.

Here is a link that might be useful: How to Seal Ductwork

In our garage is a box with a hot & cold line going into it. I think its where the ac coils are.? Where the lines go in & on the corners there are small air leaks. I just need something that can handle heat & cold & fill very small holes/cracks. I could not find what my father suggested, hvac putty, I was told fire block fb 136 would work, but I dont trust them.