Painting over paint with rust spots coming through
NASA's Kennedy Space Center is located on the Florida coast, unlike the Soviet complex located on desert steppe.  As such they have the most advanced anti-corrosion laboratory on earth. 

You've found "bronze tier" doesn't work.  That's no surprise, emulsion (latex) paints don't stand a chance against rust.

"Gold tier" is what NASA says: the best process (and the only one that really, really works) is to sandblast to SSPC-SP10 near-white metal* and then a 2-part zinc chromate primer (that yellow-green primer you see on unfinished airplanes and locomotives) then finish to suit. 

My "silver tier" has worked pretty well for me: aggressive mechanical removal (i.e. a 5" grinder with a knotted wirewheel in it).  Then I use (specifically) Rustoleum 7769 Rusty Metal Primer.  I don't know what it is about Rustoleum, but when all you can do is aggressively wirebrush, it outperforms the industrial primers I have tried. This is an alkyd, not an emulsion (latex). It will stink and be hard to clean up.  I typically brush it.

What's the matter with sanding?  Sanding can equal wirebrushing only if the surface is very smooth.  If it is at all uneven, sanding misses depressions and between/around high spots. Sanding is often a last resort of "I'm sanding because I don't have a proper wire wheel". 

Honestly, since you are painting steel, I would follow with an alkyd (oil) paint (Rustoleum's paint lines are alright).  I would never paint steel with an emulsion paint.  I have seen this done, and by "seen" I mean "spent dozens of hours removing the failed coating". 



Obtaining alkyd paint can be tricky.  There are different types of paint for different markets, and different government regulation in each market - marine, aircraft, industrial, automotive, hobby. "Architectural coatings" is the bailiwick of house paint.  Different categories have very different environmental standards ("VOC" mainly). Because emulsion paint works so well on houses, and so much of this paint is used, architectural VOC standards call for such low VOC that only emulsion paints can comply.  Fortunately most sensible governments provide exceptions for cases like yours. 

However, all your usual stores - lumber supply, home improvement big-box stores and hardware stores - 99% of their stock these days is VOC-compliant house paint.  Finding an oil-based alkyd can take some looking, and paint manufacturers use tricky marketing that can be very confusing. I check the instructions for what to use for thinner or cleanup: if it says water, it's emulsion.