Duct Seal?

Hi Everyone,
After poking aroudn my house's basement, I found that I have cantileveled floors(the floors over hang the foundation by almost 2 ft on one side). The problem I found was that the underside of the house is unsealed, although very insulationed, which allows for air movement into the house where the floor registers penetrate the floor. (tiny air gaps around the duct work and the subfloor) I saw on tv one day, someone cutting holes for registers and paint a 'goop' on the subfloor and an inch or two into the register. A comment was made it seals the register to the floor agains drafts.
What is this? Can anyone tell me a product name, and any tips on how to effectivly use it?
Thank you,
tom @ CarFleaMarket.com
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Silicone caulk once they are in place, Duct Butter otherwise.

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