Last night I was working on my 70 chevelle and notice that the rear wheel wells were coated with what looked like an under coating. It was peeling off so I removed it. While removing it the sealant in the seam between the inner and outer wheel wells came out also. My question is: Is this a factory sealant and does it need to be redone prior to painting? If so what it the sealant and were do I get it? It seems needed in order to keep out moiture and corrosion. Luckily the wheel wells are in perfect shape. Any and all help with this is much appreciated.

I would use 3M "seam sealer". It's paintable too.

You can get some new seam sealer at Pep Boys. you can put it on anytime you like, I personally would do it before painting the car.

Another vote for 3M seam sealer. I spray new undercoating after it dried a couple of days. Looks great!