ArtMetal

Alex b. here in California, i don't know were you are, most contractors use "pour stone","rockite", which are name brands for non-shrink, it actually expands about 15% to fill in all the cracks, self leveling, sets in 15 minutes type mixture that comes in 10lb box's or 5 gallon buckets. King Architectural Metals {kingmetals.com} has the rockite and manufacture says does not have any additives that promote rust. $41.95 per bucket and they ship and will send you a free catalog. {i don't work for them, just use their stuff.}
When i install steel this way i always seal the bottom with primer and paint, if project is to be painted.
if powdercoat, then it is already coated.
if patina clear coat on part installed in the set.
i have used the "tar" as in "henrys" roof emulsion on the bottom of red wood 4x4 post for wood jobs, but have't used it on steel, but this does sound like it would work there also.
hope that helps.
as far as bibbing---------wow, i trip on that all the time.
from talking to clients, other contractors, and my wife, and even God. when a job is a straight forward time and material job, {meaning something basic like repetitive panels, or basic gates, or what ever it is you do, and all it will take is cut, weld, finish, install, i add all that time and material up, add on my profit margin and that is it. a very good piece of advice i got from someone, some were said, do not bid against some else or compromise your minimum needs financially to do a job. you will only regret it. bid it on what you must have and stick to it. i always tell the client that it is not my money any way, it is my wifes and my kids,and my churches. if i have to come down a little it is only in that "profit" zone and then it still is only a little.
one more,{i wonder if there is a message limit on these chat sessions?} as far as the "custom" jobs go. my experience has been bid as before,but also add on for that extra time, which is always more then you think it will be. extra time for me is the drawings, and re-drawings,the measuring and remeasuring, the changes and extra finishing touches that only you know about but must do so you can sleep at night and not worry that someone will see it later. the change in patina formulas cause it didn't come out right. the dreaded unknowns. and last is the fact that they want you to share a part of your self that is personal, your talent and heart for better then average, and better then they can get at "home depot". most clients who want "custom" know and expect this, if it is not special, they will let you know, and if it is, they will let you know that as well.
good luck and God bless you and yours.