Bulldog, When it turns it black that usually means it has neutralized the rust. I have a bottle of some stuff that I used for my car that was buried for 60+ yeas and it worked great, you spray or brush it on, wait 2-4 hours then wipe it off with a damp rag and then dry it and its gone or neutralized. The stuff is called Burts rust remove or berts can't remember, there is a 1800 number to order it, I will post it when I get the bottle from the shop today. My dad got it at a flee market.
I would recommend brushing it on as with this chemical it was a killer on the lungs when sprayed, even with a mask the chemicals hung in the air for a long time to where you had to leave the shop when done spraying for almost an hour, Then again I had almost no ventilation in there at the time it was winter.
But I will post the # if anyone is interested in it, The bottle was like $20 and did almost my whole body. And its a 1920's touring car. So its probably smaller than your car.