@Asad - Rare spotting indeed!
Offcourse cleaning the Fuel Tank from inside is a vital thing to do, and in expert's opinion it should be cleaned once in a couple of years. As you know better than me, rust settles inside the fuel tank by the time. Specially in the case of Vintage and Classic cars. However I did not have the proper cleaning equipment so therefore I used the basic technique which I found from my research before doing the DIY. I'm sharing for all of you Beetle fan's out there so that it might be helpful.
Take a steel chain, long enough to make a solid pile. Drop it inside the tank. Shake the tank thoroughly, I know it seems tough but if you're a true Car Fan, it shouldn't be a big problem. The rust inside the fuel tank gets off everytime the chain rubs the inside surface of the tank. Shake it enough till you think that no rust will get off now. Now rinse it with water multiple times and get the rust out as much as possible. You can use Acid afterwards but I DO NOT recommend and neither do I appreciate it while doing it yourself. People may seriously harm themselves while working with it. So let some professional do it for you. This is a very basic domestic technique to clean-up the fuel tank.
As far as the paint is concerned, I love spray painting! I used Spray Cans. I know it does not give you the desired results but in a D.I.Y, I had no better choice. I'm used to with Spray Paint Cans therefore I was very comfortable handling it. I sprayed three thick coats, just to make sure the paint settles in well. The result depends on how good your hand is, and it comes with practice as I often do!
I have a plan to paint the car myself, but before that I need to find a suitable place, and a professional Spray Gun set-up. We have one in our college, just hoping that they give me permission to do so. But I'm not getting time to do that currently. Hopefully in a next few months. 
