Cleaning metal duct before sealing

I want to seal my sheet metal ducts in crawlspace, both supply & return, using both mastic & mesh tape and/or foil mastic tape, as required.
Some areas have very old duct tape. If there is accumulated dry residue from the adhesive, can I just rub off the loose stuff w/ rag and then cover the area thoroughly w/ mastic and/or tape as needed? If not, would appreciate recommendations on cleaner/solvent to use, preferably one that's not overly noxious.
For the newbie, any advice on whether mastic is easier to use than tape in hard to reach areas?
Also, are there any "rope caulk" type of mastic products that can be used to plug 1/2-inch gaps where round ducts connect? I know how to cut "expansion gaps" along the outside edge of the mesh or tape in order to curve it around the opening, but some of these connections are in hard-to-reach areas where it'll be difficult to position the sticky tape properly, whereas a one-arm-reach-around maneuver with a rope caulk might be possible, maybe followed by brushing on more mastic as needed. <sigh> my shoulders ache just thinking about this ...
-- eager homeowner
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