There is the autopaint feature. I use it to paint the entire terrain in 5-10 degree increments. Regardless, lots of hand painting still needed. 0-33 degrees is grass/dirt/sand, 33-37 is gravel/slag, 37-90 degrees is rock/cliff texture.
There's an 'Autopaint' thing, but has been broken for long time. Above is a good way around that. If you find the 'Max Brush Size' of 40 to be too small, you can hack it above that. Beware this may introduce heavy lag when painting. Edit the 'editor.xml' file in Documents/BeamNG.drive/settings: Replace both '40' values to your new value (400 is a good one, above tends to be excessively hard to use)
