I just patched my floorpans and trunk and coated them with rustolium and undercoated them to protect them. My question is , I used the duplicolor undercoat on the floors and the 3m undercoat in the trunk. Well the 3m finished alot better than the duplicolor, its thick and it looks just like griptape and kinda feels like it to. The duplicolor is kinda thin looking and in some ares when you touch it feels like it could rub off. I was wondering if I should just go over the top of the duplicolor with the 3m or if I should sand off the duplicolor and start fresh with the 3m?

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Go for it, You live this life only once.

You can probably just wipe it off with lacquer thinner. Professional products like 3M, PPG, Dupont etc. are usually much better in every way than the consumer market stuff at chain stores. It can even be less expensive if you buy them at a shop that deals in auto paint.

I would always use a "paintable" undercoating. They actually dry and can be used on the inside of the body as well. If you have any seams I would seal them with a professional seam sealer before the undercoating, you can get this at a paint shop as well in a can or cartridge that fits a caulk gun.