Denny,
When I did mine, I just used 3M undercoating. From what I could tell, the original stuff had the appearance of undercoating. It worked OK, but I had to keep the felt taped up for quite a while before the undercoating set enough to hold. 3M makes a product called body schutz that I would investigate also.
Appearance is not that important on the roof, but I wanted that black "tar" showing through the unused stamped holes in the toeboards.
Bob
