That was me to suggest using a sponge. It did work wonderfully, but you do have to really thin the paint down. The link is in my first post on this thread.
Let me be clear: On my old track I followed the advice on hoslotcarracing.com right down to the sort of paint to use, and the results were less than impressive, and I think the whole "skim it with a razor" thing was the reason.
You really have to be methodical about cleaning the rails off after, and testing each piece as you go. I used a blade, an old butter knife, some fine grit sandpaper, and a fair bit of persistence.
Set up a terminal track, and after you've gone over each piece, attach it to the terminal and run a car on it. Inch the car , stop, go, forward bit by bit. If it stalls at any point, some more rail care may be needed.
I'd add a caveat. painting the track is a lot of work. Don't let anyone fool you. The painting is the easy part. The popular advice about simply skimming the top of the rail with a blade isn't enough in my opinion. I believe you need to get the sides too.
So you really have to be sure it's worth it for you to have a grey track. In my case I think it is. I have something very particular I'm trying to model.
But if its just a whim, I'd say think long and hard.