John - I've used the 3M undercoating stuff in the rear wheels. It comes out rather chunky if I'm not mistaken. How do you propose to use it for sealing the heater box, ground straps cables, etc to the firewall? Seems like it could make a mess.
For the 3M sealer I used above, I just squeezed out a dab and apply it, using latex gloves. Pretty easy. For the heater box, I applied a bead on the inside channel to the spec called out for in the A.I.M.