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 beerstone that needs to be dealt with. Usually a byproduct of local water chemistry/ brewing conditions, this can be a pain in the butt when it happens because the agents and chems normally available to us home brewers don't really tackle it all that well.
If you have a local brewery there you frequent / haunt, 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. Just be careful with the stuff and not getting into your eyes!
Here is another good link to check out ..