I have no opinion about Salt Away. I've heard good and I've heard bad. I have never personally used the stuff.
Get what you can, as deep as you can with mechanical means. Scraping, chipping. blowing out.
BUT! Scale will actually embed in the metal and if you get too aggressive, you can start chinking out pieces of the engine. The worst part is what you can't see and can't get to. It's in the head and the block. Sooooo....
After that, I would try this; CAUTION...THIS IS EXPERIMENTAL...DO SO AT YOUR OWN ...whatever. You get the picture.
Prestone makes two products for cars that are HIGHLY effective. One is...COOLING SYSTEM FLUSH and the other is...COOLING SYSTEM CLEANER. Problem is, they're made for cars. Oh, I already said that.
Since the stuff is for cars, they designed them to be used with systems that contain 1.5 to 2.5 gallons of water. I have a unique can I can put the lower end of an outboard in and fill up to the water intakes. But, even though it's a tight fit and skinny, it still takes around 5 to 6 gallons to cover the intakes. So, the "recipe" has to be doubled. The good part is, these products aren't all that expensive.
You use the COOLING SYSTEM FLUSH first. Just follow the directions but use enough to treat the amount of water you have in your tank or you will just be peeing in the breeze. FLUSH and rinse with the T Stat removed.
Then, use the COOLING SYSTEM CLEANER. This stuff is powerful and breaks down scale off of the metal. Same precautions as with the flush then rinse thoroughly. THOROUGHLY!
When you open up the housing to reinstall the stat, it should be sparkly clean. Note that I say "should be". Why do I say "should be" you ask?
It's because I've never done this. See warning above. I intend to do this but I have NEVER ACTUALLY DONE IT YET!
Soooooooo,...I don't know...well there are many things I don't know. As you have probably already figured out by typing with me for days and days.
Anyway, What I don't know about this is what this stuff will do to a rubberized water pump impeller. I have used this stuff to clean AUTOMOBILE radiators and cooling systems for years and it works GREAT on those. Turning old, scaly, clogged radiators to shiny and gleaming clean, almost new looking inside.
But, what it will do to the impeller or, more importantly, what it might do to the seal in the power head where water enters the block from the water tube, I just don't know.
I had intended on doing some "trials" on this stuff and how it affects rubber components but I never got around to it what with meeting all the new friends I have now since I got old. Mostly MDs.
So, you're on your own there but I thought I'd throw it out there in case you might want to be the guinea pig. It would save me a whole bunch of time and effort. And, if you won't do it for me, what about mankind?
Alright, SO WHO"S NEXT HERE!