glider refinishing and airbrake caps

What products have been used to fill the aluminum caps of schempp-Hirth gliders during a refinish? are repairs using just a thick coat of primer surfacer like prestec? or is there something else used. the aluminum surface of my Std. Cirrus is a good .040 or more below the wing surface. I would expect something with some flexibility might be used but the original stuff is pretty crispy.

Thanks,
Scott