How often do you reapply the gel gloss?

Yes, thanks for making that point, magic erasers are abrasive and I would not use one on a good smooth surface. However, in this case it has the opposite effect, and is actually smoothing the rough surface.

Consider sandpaper grit sizes, if one was to sandpaper a surface such as raw wood, with 20 grit it would roughen the surface, but if 300 was used it would smooth the surface of the wood. So the smoothing or roughening of the surface is relative to the coarseness level of the abrasive to the surface.

In the case of the old tub which has a roughness level of approximately Ra Micron (µm) 0.30 - (320 grit) vs. a new tub with a level of Ra Micron (µm) 0.10 (very smooth), abrading it with a magic eraser (3000 grit) would in fact smooth it, making it closer by a little bit to the original surface.

Your statement “Once the porcelain over the iron is damaged, there is no seal or glazing or anything that fixes that.” is an assumption, I have repeatedly found it is best to avoid the terms “never” and “always” because they often prove in the end to be incorrect. There are many products, processes, developments, and solutions being continuously developed that were never thought possible. To just give up at the start is not the best approach in my experience. If the claims of the science behind some of the listed products is actual, then it would confirm my point. However, that said, there is also a lot of snake-oil bogus products around, and for me, the only way to verify the claims are to test them or ask people who have done so for their results, which brings us back to square one (read the original question).

Reglaze. We did that with ours. It was great and looked like new. We didn't heavily scrub it so the finish didn't wear off. It cost us about 500 bucks eight years ago. The person doing the work is critical. Our guy also fixed an area that was cut out with bondo. You would have never known, and it was along the edge so didn't get heavy use, just a visual . . .

They will tell you what to clean it with. Our guy came well recommended from the local plumbing supply place. We lived in a neighborhood with lots of old original tubs, he did it a lot and specialized in it.

JuneKnow: You are correct. Sandpaper as well as magic erasers do remove material. However, in this case, because the high surface is already gone completely, the abrasive action is smoothing the hills and valleys creating a glossier surface to make the surface more resistant to future soil, this is on a microscopic level and does not really effect the tub significantly as most of the material being removed is dirt. Just as sanding a board does not significantly reduce the strength, durability, or thickness of the board. Similarly, using polishing compound on an auto finish removes paint, but again this is minimal and does not negate the benefits of doing it.

I only clean porcelain with diluted peroxide. If you have a brown spot it is rust. Chlorine rusts the cast iron beneath the porcelain glaze. I had tons when I first moved in, the last owns used a bleach product to clean. I soaked a face cloth with full peroxide and left them over the stains to get rid of them and there after just use peroxide for regular cleaning. Gel gloss gives me awful headaches so I do not use it and the only way to fix the etchings ( grey areas) is to replace the tub or repaint the finish.

I cleaned a tub with bleach once and was shocked that it immediately produced a huge brown spot when the bleach hit the surface, I think it clung to the grime. I tried everything I had to remove it and no luck. Finally, after reading a tip on the web, I sprayed it with standard 3% peroxide and amazingly the stain began to disappear (without scrubbing it) in about 60 seconds and was completely gone after 3 applications. I thought my tub was a goner. The tub I am working on now is 80 years old and completely white (no rust or grey spots), so I would like to get another 20 years out of it by giving it some TLC and sealing the present surface. Some of products mentioned in the OP claim to create a permanent bond, However, for some reason I am skeptical and am looking for real time users to verify the results.

Reglaze it. That is the only way to go to have a satisfying outcome. I had it done two years ago for $800. It looks great.