Ive been experimenting with masking fluid and candle wax to create vapor trails in my skies. Ive found that a masking fluid marker pen is the most manageable way to apply this stuff. For the softer trails I rubbed some candle wax across the trail as well. Once the sky was dry I used my 'eradicator' brush from Rosemary and Co to softly diffuse the edges of the trails. This brush is so cool - I would recommend it. Regards painting the sky I wet the paper twice. Firstly back and front with a sponge and then leave it to soak into the paper. Then ten minutes later sponge it again and wait until the shine goes. This gave me a lot more time to apply the layers in the sky. My approach is to work on the 'heavens' first (the yellows, reds and turquiose) and then slowly move forward into the darker clouds. With every 'layer' use a slightly thicker mix. There's still a long way to go, as I had to go in once dry with some extra washes - I would do this differently next time around, but i feel like this approach has potential.