it isnt tar it is sealent/cocking to seal the car i believe. if you are removing the floor pan make sure to put cocking in the same spot if you arnt and it is rubbery in feel then dont remove it it helps seal up your car.
Great choice, thats what most recommend. Make sure you prep the floors really well or else it wont stick. You'll regret it later. I used self etching primer with a few layers of paint on top just cause it was easier for me. Also make sure to cover up everything with newspaper/trash bags cut open/whatever you got.
If the car still has the original underlayment, it is bitumin(sp) tar and it is petroleum based. I used to buy it by the roll as it was essentially the same as original underlayment.Scraping is the answer and would also use mineral spirits which is also petroleum based. Better for removal of residue than lacquer thinner and also easier on the eyes and lungs plus less flammable. Using it with a wire brush is useful, also. For a final cleanup would then use simple green or similar stuff. This will aid in better adhesion for paint.
Are you talking about the caulking or the thin layer of black tar like coating.
For the tar-like coat I sprayed it with eastwood undercoating remover and let it set. Then I took a greenie and sprayed Eastwood PRE paint prep on it. Then a little light scrubbing loosened it right up, wipe it with a rag.
For the goo (seam sealer) I used a blow torch to heat it up and a putty knife to get most of it. Then refer to step above for the remaining stuff you can't get.
I used a small torch where I could, and a heat gun where I was concerned about overheating or fire. Once it's hot it scrapes off clean down to the paint if you use a razor scraper or a good putty knife.