How long before water damages the surface of my steel bathtub?

How long before cold or room temperature water damages the surface of my steel bathtub?
12 hours? 24? 96?
When I bought the house I thought bathtubs were invulnerable. My tub was incredibly uncomforable as a tub, couldn't lean back, so made cushion out of 4 inch thick foam rubber, and then it was just fine.
I left about 10 inches above the edge of the tub, minus a part I carved out for my head.
I also made a skirt about 10 inches below the top edge, by cutting away the two inches nearer the wall, and by cutting horisonally at the sides so it wrapped around inside the tub.
And everything was well. Until a few months later, I moved the foam and the surface of the tub was damaged at the bottom. The water would drain out at the end of every bath, except for part of the lowest inch or less. That never drained and that's the only place the damage was.
Now I have occasion to partly fill the tub and leave it that way for maybe 24 hours and I'm afraid I'll damage the tub, and since the whole bottom of the tub is under water now, I"m afraid I'll damage a lot more than part of an inch. I know an hour won't hurt it, but how long have I got until it's damaged?
Or was there something about being between the wet foam rubber and the tub that caused the damage?
P.S. All later cushions were above the top of the tub only and were just as comfortable. Alas, I wish I had figured that out in advance.
An electric knife works well on foam rubber. A regular knife does a bad job and a razor does a mediocre job.
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It was probably not the water, but either some residue from the foam or trapped minerals that caused the problem. In a porcelain coated tub, water should be able to sit in it for months. If you have very hard water, some of the minerals may be deposited on the surface.
If you still have stuff on the tub surface, try CLR or Lime Away to get rid of it.
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wrote:

Well, I hope you are right. Thank you. The stuff is grey and not the colors I've gotten when I let a faucet drip (which I did remove with something like CLR), but I will try CLR and see what happens.
I'd be very happy if you are right, and it would more than make up for my being stupid.

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wrote:

My friend suggested this too, a year ago, and even though he's smart, his smugness on this occasion got to me, and maybe that's why I didn't try it. I had tried to feel with my finger nail where the ridge was, and it didn't seem like there was one when going towards the crud, or away from it for that matter.
But after you suggested it too I tried it, 4 times, let it sit for 3 minutes the first time as suggested, and much longer the other times, but didn't see any change. (thought I saw change in one place the first time, but there must have never been anything there to begin with.
I'm going to try it a few more times, or get limeaway, but I don't think this will help.
Thanks a lot anyhow.

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