If it's rusty, you'll need to rinse it, clean it, pickle it (also known as chemical etching) and then seal it - or - if you can find someone who has a tank tumbler - you can just put some aquarium gravel and water into the tank and let it roll around for a couple of days to clean it. Some people even use roofing nails or ball bearings and water in the tank instead of gravel. BTW - the tank tumbler is a machine which rotates the tank around all three axes at once (think - portable cement mixer). The action is slow and fairly gentle but two days of that motion, along with a couple of rinses, and the tank will be shiny and clean inside.
During this past fall, I did that tumbler routine with my XS750 tank for my cafe build (see the thread '81 Special to cafe build" - there is a video of the machine working and it did an excellent job. Once it was clean, I just sprayed in a little WD40 and sealed it up with the petcock and the fuel filler cap. Once I start to use it the tank should stay clean as long as I keep fresh fuel in it.
Last winter, prior to finding the local chap who had built his own tank tumbler, I had a rusty tank on my '76 Standard after it had sat empty for a long time. I did a fair bit of research and I found that the best stuff to clean and seal a rusty tank is called POR15.
A lot of the bike shops around here used an older chemical treatment called KREME - but apparently it can be dissolved by the ethanol which is added to many brands of gasoline these days and so it will eventually flake off and clog up your fuel system. If you look around, you'll find that a lot of people have had very unhappy experiences with KREME - but POR15 seems to have a very good reputation.
I would suggest that you check out the POR15 website - it is the same stuff that Jay Leno uses on all of his big-dollar vintage cars and he did a very convincing testimonial on the POR website for them. You can get a POR15 motorcycle tank cleaning and sealing kit that includes all of the necessary stuff and it did a superb job on my '76 tank. One of our local autoparts places sells the kit for around $70 I think, although I got my kit on-line.
A couple of things to note about using POR15:
- sealing the fuel filler hole and petcocks holes is not easy. I found that no type of tape would do it. I used a rubber bung (like a cork) from a wine making store in the filler neck and I made a couple of patches out of old hotel plastic keys (sort of like credit cards) for the petcock holes and I simply fastened them onto the tank with the petcock screws.
- the chemicals you'll be using are pretty strong so follow the instructions to the letter and don't spill them on anything you care about - particularly the sealant which is a silvery liquid about the viscosity of maple syrup. It is impervious to any type of paint thinner or stripper.
Personally, I liked the tank tumbler method because there were no nasty chemicals and now that I know a local guy who has one, I'll always use it for dealing with an old fuel tank (assuming my wife ever lets me buy another old bike that is). However, I was perfectly satisfied with the job that POR15 did on my '76 XS650C fuel tank, so I wouldn't hesitate to use it if you don't have a tank tumbler handy.
Which ever method you use, it is obviously important to ensure that the fuel in your tank stays clean and that no crud gets down those little hoses into the carbs. Once you get it cleaned out - keep the darned thing full of clean fuel and use it so that the fuel doesn't go skunky and cause problems in the future.
Cheers,
Pete