Dopped in and re-sized, here I was trimming away the source background
step 3 of 5
Here I used the eraser tool with a standard grass brush on the car layer to pull grass in front and see if it could look natural. I would add more later after merging the car layer with the rust layer.
step 4 of 5
Here I am adding rust onto a new layer with various spatter tips and soft rounds. I greatly varied size and tone. I also went around earlier and painted out all of the gleaming highlights.
step 5 of 5
The final step was to "add" more grass with the eraser on the car layer and do some more touch-up on the rust.