Dee, I have one of those hard to clean tubs too.
My suggestion is to spray it down good with some Clorox Cleanup spray and let it sit for a bit, go in rinse first and then scrub with soft scrub. Also, did this once when I had run out of the clorox cleanup, I had some of that toilet bowl cleaner w/bleach, the thick gel that clings, I squirted some in the tub, coated the tub with it, let it soak and it was sooo clean.

I have to admit, I am curious about the lemon and salt, because lately I am thinking I am tired of harsh chemicals and want to try more eco friendly ways of cleaning.