Steel Support Painting Part 1

I've been meaning to get these steel support beams painted for 2 summers.  Last summer it was just to wet, and earlier this summer was just too cold.  So I finally got started yesterday.  It's not going as quickly or as well as I'd like.  So if anyone has suggestions, I'm looking for help.

Starting Out

Friday I stopped at the Gear Shed and purchased some Ospho, which is supposed to convert the rust on the beams to something or other, so I can paint over the top.  Yesterday, following the instructions from the guys at the Gear Shed, I scraped down the rusty beams with a couple of scotch brite scouring pads to get all the loose paint & rust off.  This went fairly well and I used a brass brush on the weld seams & other hard to reach areas.  Then I mixed the Ospho about 40/60 with water in a spray bottle, and sprayed the mixture on the beams.  As you can see, it looked to me like it was working.  After a bit I was getting the white powdery residue that I was expecting.

Rust Close Up

However when I looked at it this morning, there was a LOT of rust still visible.  I thought I'd have to put on a 2nd coat.  And I'd used up all my Scotch brite scrub pads so I got out the sanding paper.  Every pass with the sanding paper revealed rust.  So I spent hours sanding & scraping and re-applied the Ospho full strength. 6 hours after I finished, it still not dry.  So the rust conversion should still be going on.  But I can see orange rust spots all over, though not as many as this morning.

1st application of Ospho

Sunday Morning

Close up white residue

I'm now wondering if the beams were covered with something besides scratched paint that didn't react as I expected with the Ospho.  Anyway, if you have any ideas on how to proceed, please let me know. I really don't want to spend another 6 hours tomorrow sanding and re-applying the Ospho, but I don't want to paint over the rust either.

Sunday afternoon after 2nd application.