I just solved the problem of cleaning the fiberglass pop top as I was
readying the 81 Westy which I now have for sale.  This van had never had
it's top cleaned and had been sitting under a lilac bush for the last 4
years.  About the only natural item not on the roof was a nest full of baby
birds.

First, I used Simple Green degreaser/cleaner with a scrub brush, and lots of
water with a hose.  This was tedious, but removed all the dirt and other
junk.  However, the top still was grey colored.  I then used straight
laundry bleach with a plastic scrub-type sponge.  It was AMAZING how easily
and quickly the color went to sparkling white.  (Thanks to Listee Roy
Mathers for the sugestion.)

If your top is not too bad to start with, I would suggest just using soap
and water first, then go right to the bleach, which would save a lot of
effort.  You may wish to use rubber gloves with the bleach.  I worked with
and without gloves, without too much difference to my lovely hands.
Remember that small drops of bleach will likely splash onto your clothing,
with expected results.  I tried to make the splashes form a VW pattern, but
my artistic abilities were not up to the task.

Unless you are tall enough to get a basketball scholarship, the use of a
step ladder is highly recommended.

Overall, the results will astound and please anyone who makes the effort.

Happy cleaning!

Budd Premack
86 Syncro, 84 GL(for sale), 73 Super
Minneapolis, MN (Land of Sky Blue Waters)