I've never used coffee grounds for scale (but I will if I ever have too)
I have used it to get a sick rain tree completely healed up. I just threw the grounds around the base of the tree and watered it in or before rain. I think it had soil nematodes or something else but its flourishing now.
Year after year the tree leaves would dry and curl up and fall off. This tree planted about 4 years looked ready for the dump but no more.
Below are some links to look at quickly and get an idea (or of others) using coffee grounds. Don't be shy to ask Starbucks or any coffee shop for their grounds to start a big project. They say they are good to give it away. Its also good for compost and compost people are always trying to get their paws on it. Or drink lots of coffee!
( one tree a day )