any tips on applying a grey wash

I dont get what you mean !! you use grey wash for shade ??

yes, tips on even adding grey or light black to large areas?

if your using grey wash to shade then my advise is use a minimum of 4 pots to give you a gradient in shade to make more realistic, pot 1 all grey wash pot 2 75% wash 25% witch hazel then 50-50 the 25% -75% this will give you the best gradient to go from black to nothing laaving no lines of different colour you get it to just fade and vanish to the eye

My first try at shading didn't go as planned. Next time I'll try a lighter mixture first just to see how it goes from there.
I mean I can go darker not lighter lol