This has been discussed before. A search of the forums should turn up a number of suggestions. A couple of them might involve putting some small stones, bolts & nuts, etc. in the tank and shaking or rotating it to loosen up the stuff inside. Some folks have used muriatic acid to wash out the tank, others have taken them to a radiator shop and had them cooked out.
There are also a hundred different opinions as to what kind of coating or liner to use, or whether to use one at all.
Those are just some of the methods that are out there. As this thread progresses I'm sure you'll hear them all, plus some I left out.
Good luck.
