I had hardened depostis in mine when I did the first coolant flush that seemed to be cement blobs painted brown. Those things just refused to be cleaned and held on to he inside of the tank shrugging off attach after attach I launched on it with everything from full strength degreaser to CLR to paint thinner. None of it did much, most of the hardened deposits remain to this day in fact.
But now that I know a secret weapon to use in eradicating them, I just might have to do another coolant flush...it would be about 50K miles too early for the coolant if I changed it now, but if it means finally defeating those immovable deposits inside the overflow tank, I would be willing to endure the extra cost to secure the victory over the brown cement!