Just came across this excellent article on prepping a new or used has tank. It's a keeper.
Ron
QUICK TIPS - FUEL TANK RESTORATION - By Glenn Ring
Sooner or later you will need to replace or restore the gas tank in your Volkswagen. Over time, water collects on the inside from bad gas or condensation, and will rust the tank from the inside out. You will notice the smell of fuel when you fill the tank, or make turns. If the tank is beyond repair, with numerous pinholes or larger rust holes, it will have to be replaced. All of the generic aftermarket tanks that I've seen are of poor quality. This means that you may have modify them to make them fit. Another alternative is to find a used tank, but you might be in the same boat again in a few years.
If you want to save the tank before there is a problem, or if you are replacing it with a used tank, you should take the time and apply a sealer. I recently restored a tank using a product form POR 15 (www.por15.com). (POR 15 products are also available from a Cal-Look.com supporter, www.aircooled.net - Ed.) For those who don't know, POR 15 has an entire line of rust repair paints and is well known in the vintage car community. I decided to use their "US Standard Fuel Tank Sealer".
After draining all of the fuel I removed the tank. There was some rust on the inside so I took it to a local radiator shop to have it "boiled". This is a process of submerging the tank in an acid solution to remove any loose rust, sludge and gummy build-up. After this process, the tank comes out nice and clean on the inside.
I wanted the tank powder painted so this was the time to do it. If I did it after sealing the inside the heat of the curing ovens would damage the sealer. If you are painting it with spray cans or a brush, you can do it after sealing the tank.
POR 15 sells a special prep called "Metal Ready" that needs to be applied before the sealer. It etches the metal and leaves a zinc phosphate coating. You need to close all of the openings, except the sender opening, with duct tape. Then pour in the Metal Ready and tape the sender opening closed. Slowly rotate the tank, making sure the liquid coats the entire inside of the tank, moving it back and forth, side to side and upside down. After about 30 minutes, drain the metal ready and wash the inside with warm water. Drain the water and use a hair dryer to totally dry the inside.
Now you are ready to apply the sealer. Tape all of the openings closed again and pour in the sealer. Rotate the tank again, making sure that the entire inside surface is coated, and then drain the sealer after a few minutes. Leave the tank for an hour or two to make sure that all of the sealer drains out. It takes a few hours for the sealer to harden and 96 hours for it to fully cure.
Now the tank is ready to be reinstalled, and should last for another 30 years.
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