The two masking methods I use are layer masks and clipping masks. I think I used clipping masks for this – Start by painting solidly the silhouette of the entire object, then make a new layer on top, right click, and "apply as clipping mask." That way, whatever you paint on this new layer will only appear on where there are pixels on the layer below. Hope that helps?
You nailed it ! and the design is great ! The only thing bugging me out is that the front right tire is a lot more "round" than the front left, maybe it should be flatter, but other than that I love it