I've used an ultrasonic cleaner with dawn to remove light surface rust on tools before it shouldn't affect the stonewash finish or anything. I would personally just pour dawn on the liner and scrub it with a brush than put it in the ultrasonic and let it go to work. My unit was about $20 and works quite good with just dawn and warm water. Just be careful of what you put into the machine as it may damage/destroy it, and don't put your hand or any body parts in the machine while it's running it's bad for your health to say the least.
I feel like I have to put in a disclaimer as I've been pushing the dang ultrasonics a lot in the last few posts. But truth be told I just love the thing, and been using it to clean a lot of things since I got it.
Beyond this I am quite certain quite a few members are more knowledgeable about your problem and will chime in about various solutions, so the best of luck to you.
