If you have a stubborn scaly substance on your stainless that won't come off with traditional cleaning methods, you may have a case of beer stones that need to be dealt with. Usually a byproduct of local water chemistry/ brewing conditions, this can be difficult when it happens because the agents and chemicals normally available to home brewers don't really tackle it all that well.
If you have a local brewery where you frequent you might ask them if you can have some nitric acid (and one other, they’ll know) to remove beer stones. Chances are they might have similar water chemistry with you as well and could help you out. Be careful with acid and do not allow to come in contact with your eyes
Here is another good link to check out. Removing and Preventing Beerstones