Vorgellack (T35) is an old type of gel that that we used to do final finish on gliders up to 2003. Vorgellack is the most prone to cracking caused by UV damage. T35 is less durable gets yellows faster than PU but is cheaper and easier to work with. The exposure to UV and a damp atmosphere results in a colour change. When cracking damages the surface as these cracks deepen they propagate into the structure damaging the top fibreglass layer. T35 provides a good base layer for 2-pack paint finishes.