I agree with red58 on this. You should seal it after you clean it. Eastwood Comany sells a good kit if you are looking to do it yourself. The Por15 kit also comes with high recommendations. Probably can't go wrong with either one.
If you are just trying to clean your tank out, then go down to your local home improvement store and buy some acetone. Dump out the old fuel. Then pour in some acetone and drop some nuts/washers/etc. in and shake it. (Note: just remember how many nuts/washers/etc. you put in there so that you can make sure you get them all out). You might have to flush and repeat a couple of times.
Hope this helps,
