Hi Rolf,
The thread on this subject is dead but I was interested and wanted to throw in a couple of ideas.
The need to sand a peel ply surface before secondary laminations - I think this advice got started because some builders, to save money, used ordinary nylon or dacron broadgoods that were not properly heat scoured and finished for use as peel ply. If you stick with the well known manufacturers there should be no need to sand. In the states the manufactuers are Precision Fabrics and Airtech International.
Peel strength - this also has a lot to do with the type of epoxy curing agent. Most of the retail brands that are 2:1 or 1:3 and sold for wood boat building are either Polyamide of Cyclohpetic (spelling??). There are also used in the coatings industry and for industrial adhesives. They tend to have flexibility and thus good peel strength. On the other hand, a high end structural curing agent based on DETA or TETA chemistry is stronger, stiffer but more brittle. So, it will more readily delaminate under impact loads.
As with all boatbuilding things, it's a compromise.

Kevin

Thanks Kevin, all very interesting! The 5:1, 3:1 and 2:1 formulation issues was something new to me and might explain why some are more sensitive to e.g. WEST (5:1)?

For the peel ply I am uncertain. Your opinion weights heavily as you have loads of experience. Others, especially the amateur aircraft builders, say to sand anyway. Did you get your information from the cloth/epoxy manufacturers or through experience? I know the epoxy manufacturers said peel ply removed the need to sand, but they pulled that statement back as far as I have heard?
At least I now know to wash and sand thoroughly if I use regular nylon as peel!