Cleaning with true saturated steam, or as Harry said, having a radiator shop boil it out, are both excellent ways to clean your tank. It is getting more and more difficult to find a radiator shop that will work on gas tanks, which is why I now have a steam cleaner here. After letting this most recent tank steam for five hours, it was clean enough that I could weld on it. That said, never ever ever weld on a gas tank, pressure vessel, or anything having contained a substance with a low flash point unless you absolutely know what you're doing.
But yes, the steam cleaner will remove all the goo, sludge, loose scale, whatever you may have. If you still have some scale that won't come loose without some mechanical cleaning, drop a chain inside and let it slide around to knock it loose and then rinse it some more. The type of rust I'm talking about is as you would have on a hood or the roof of your truck that might be covered in well established surface rust, is what I'm describing as being stable. Your tank sitting empty is what most likely caused the scale you have, generated by the moisture in the air. If it had been left full of clean gasoline, it wouldn't have done that.
There is no reason to add a second filter.
Here are the contents of a '32 Buick tank that I steamed out last year:
You would think the inside was shot from seeing that, but it looked like this afterward:
The color of the sludge is just the color it naturally turns with age. You've seen this if you've cleaned old orange gasoline out of your lawn mower.
The best suggestion given so far may be msgross' to just buy a new one. By the time you pay someone to clean it, it could get pricey depending on what they charge. I'm just used to dealing with things you can't buy new tanks for.