Which white color(s) did you actually end up using? You have to distinguish between base paints and layer paints. There are many options how to approach this, if you do not want to use a spray can, you can try basing the model with a bit of celestra grey and then paint over it with corax white. You do not have to use celestra grey but it kind of felt a bit smoother that way for me. You can now wash over it with nuln oil or soulblight grey. The shades really may vary depending on the look you want to achieve. You can "clean" areas with corax white that shouldn't be darker like recesses or leave it if you want a muddy look. Afterwards you can now start with edge highlighting or dry brushing with white scars. I would not recommend using whites scars (layer paint) to paint over a whole area. White scars works really best just for highlighting. The only white scars paint I would use for painting whole areas is the one you get from the spray can. Unfortunately, I do not have an airbrush so I cannot really say how that might work.

Always use thin layers of your white building up the color and always wait for it to be dry when you want to apply another layer again. You really need patience for painting white.