help with angle iron's

[DaveH]

Dave, Wow! that post is so easy to read and understand.   

We really appreciate the effort you went to  

Unfortunately there is sand paper, sand paper and sand paper and it can vary considerably in it's quality.
The type of grit used, how much grit there is per square inch and how well it is 'stuck' to the paper.
I don't think you are doing any thing wrong, it is just the quality of the sand paper you are using.
I also don't think you are using an excessive amount of sand paper.
One thing that may help (a little) use lots of water -helps stops the clogging and keep wiping the 'dirty water' off the leg.

For your first time - you are doing great
 
DaveH