Is an old type of gel that that we used to paint gliders up to 2003, we did this because it was the same Gel that LS, Schempp and Schleicher were using - they still do.
Vorgellack is the most prone to cracking caused by UV damage.
T35 is less durable gets yellow faster than Schwabelack 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 is "softer" than Schwabellack and provides a better base layer for 2-pack paint finishes.