Will certainly try phosphoric acid - sounds like the silver bullet I need. At first I tried a plastic scraper after soaking with 9% vinegar - after a short time (approx. a minute) the edge was blunted and no longer of use. My first expectation was the acid would soften the deposits to facilitate mechanical removal - was not the case. The acid dissolves the deposits. Upon placing the small piece that is under the inner drum in vinegar, bubbles appeared and streamers of dissolved material were visible.
POR-15 will be applied - I may be too particular in wanting all deposits removed and will pursue that objective. Having seen photos of tubs in other posts that appear as brand new - that is my goal.
Thanks again to all for the benefit of your experiences!