The poptop is a big job to remove.Big Big Big. You will problbly need a
new canvas. Maybe youcan dothe job good enough keeping the top on.Sounds
like you have an early Vanagon with a white top.

A bathtub scouring powder like comet works good.Use a strong solution of
clorox with this powder. Dip the sponge in the clorox and  sprinkle comet
on the sponge. Use gloves.Carefully clean a small area. dont press too
hard. Let the clorox do the work.It takes overnite for clorox to lighten
up the top.You will be amazed how good this will work.

The inside floc is a durable finish,even though it looks fragile. It can
be cleaned with a clorox solution pretty good. I had mold spots and the
clorox clobbered them.

When you are done, you will probably need new stainless steel bolts that
hold the to the Vanagon. Your clean looking  top will demand them.I happen
to know of somebody that sells them(me).Go to
http://stores.ebay.com/werksforwagens