The stock tanks are nearly impossible to clean without cutting a large hole in the top because of the partition. But, before you put the sealer on you'll want to clean it out as best you can. I suggest lacquer thinner for rust, and if anybody got a suggestion for the tar/old gas my ears are open!

If the car has ever sat it probably also has cruddy gas in the bottom of it. I run enough first gens through the shop that have tank problems that I keep the driveway paved off the crap inside...