I am just asking generally speaking, what products might be used. My user name tells I don't have a mustang.
OK - for some years and plants I've used black smooth elastomeric sealant from the chalking tubes - applied then spread with a thinner dipped brash with 3/4 of the length of the bristles cut off. For other plants and years - I've used Sludge applied with e brush or Wurth and Lord Fusion shot with their applicators/guns.
Just depends on the final look I've going for since some were just a bead of sealant- sometimes sealant applied and rubbed with a solvent dipped rag while others were sprayed
The trunk drop offs and wheel houses were only sealed where the two came together in the trunk and the trunk drop section of the passenger and drivers side trunk floors were not sealed except, some plants and times, on the outer, inner (gas tank side) surface over the primer sealer, over (well that was the idea) the spot welds between that panel and the quarter panel. That was normally applied from a gun (chalking) then spray with a wet rag to about 3/4-1" wide like the rocker to floor seams most years
