Sorry Steve, I missed this.
As the original question was about brush marks, here's my method.
Thin your paints. I haven't a clue re. water/ paint ratios. I just add a bit of tap water until I get the thickness summat like right. Thinned, but with plenty of substance to the paint. Practice on a bit of spare plastic.
Use as wide a flat headed brush as is practical. Try and use one with softish bristles.
Get the paint on a quickly as you can.
Work from the middle of the piece to the outer edges. Do it the other way and you will get 'Runs' on the edges. With aircraft, lay off the paint in the direction of any wind flow.
There will be streaks. Don't worry, just get the surface smooth.
Don't fiddle! Leave the paint alone to level out.
Once dry, apply a second coat. Don't be tempted to thicken your paints. The streaks will disappear after this or perhaps a third coating.
As you will have read, folks have different methods. All I know is the above way works for me, all the time, every time.......
I hope this post will be of some help.
Ron