There's cleaning and then there's corrosion removal. For cleaning, I use 409 which is apparently very similar to SP's own cleaning solution. Maybe they just re package the stuff and mark it up 1000%.
When I buy an old reg and there's corrosion, I'll use an acidic solution to remove the corrosion. I use a very weak muriatic acid solution, more or less equal in acidity to vinegar. If you have an ultrasonic, you can use 50/50 vinegar and water, but I don't so I've found that the strong solution works much better. Once I've rebuilt a reg like this once, I don't have to use any acid, or at least haven't had to yet. What I have found is that with significant corrosion, the corrosion has already damaged the chrome, and after the acidic bath the brass shows in the spots the corrosion was.
If you never found a source for the IP shims, think about a brass washer that's .012 thick; you'll need a micrometer to measure it. Personally I think that would be a big improvement over the plastic shims, which I find unreliable in terms of how much they raise the IP. I think they tend to flatten out and the brass would really provide a better surface for the spring.
