A sealed rust patina, I find very appealing and it can be especially useful in outdoor applications. I plan to use it on the outdoor sunflower sculpture on which I am currently working.
In this project I have used the usual 14 gauge material to built the hollow formed stem components but they are not yet fully assembled. Up to this point the pieces are less than 5' in length, which is the maximum my tumbler will accommodate.
I decided to go ahead and start the rust patina process and let it work as I studied how to complete the project and to store all the pieces outside which gives me that much more shop working space and lets the weathering work on the patina naturally.
The surface was cleaned with a degreaser such as acetone or lacquer thinner after tumbling. Then I applied muriatic acid (hydrochloric acid) with a chip brush and immediately sprayed the surface with hydrogen peroxide in a plastic spray bottle.
This will usually produce some rust color almost immediately. I'll return to check on the progress every few minutes and probably brush with the acid another time or two over a half hour period. By then the surface usually looks pretty evenly rusted and after that I use only the peroxide spray another couple of times. I leave the pieces outside to weather until they are ready for final assembly.
The last step will be to apply a sealer when the project is finished and the patina is evenly established. I will use fast drying satin polyurethane as the base coat. Several additional coats of the same can be applied or another type of satin sealer which is compatible with it. I have experimented with several finish combinations to find what works for me and what doesn't.