Use a cheap 2" brush that has fairly stiff bristles. If there is an a/c
supply house near you, see if they will sell you a gallon of duct
sealer. The stuff HD and Lowes carry is not as good as what is used in
the trade, and they get about as much or a small container(quart?) as a
gallon costs at the supply house. There are several brands, all about
the same, that wholesale for about $11-14/gal, though they may charge a
homeowner more, assuming they will sell it to you at all. They will also
most likely have the brushes too. BTW, you are probably thinking the
stuff is thicker than it really is. It is about the same consistancy as
drywall mud. Good luck Larry