Why seal a rust finish? 

Can use nyalic clear.  

Or any matte polyurethane should work. 

I guess i can see why you would seal it if you live in a humid area. I'm spoiled living in the desert.  Here metal rusts very very slowly.  

jia-x what desert you living in?

The Sonoran.  Phoenix. 

Glad you didn't Say Scottsdale....lol. I would have put you in an pink jump suit and cowboy hat  wearing boots with your pant legs tucked into the boots.  You would have your Glock hanging from your side with and extra clip or two.

I actually recall finding a product produced in California  which has  a matte finish.  The difficulty with applying any of these materials is you need to apply it in a shaded condition. Corten Steel tends to get really hot in the sun.  

A friend of mine uses a paste automotive wax on steel on the indoors and it does give it a nice rich, deep look, of something you must touch.

Acrylic sealant, often used for cars, is not a bad option. It is based on a thick, liquid polymethyl methacrylate.

Acrylic sealant easily adheres to the metal surface and has high strength but mediocre elasticity. Therefore, in the case of high loads or strong vibrations, it can peel or delaminate. Therefore, acrylic should be used only for sealing joints in stable areas of the vehicle. I have met people who use this sealant in combination with Scrooz Fasteners. It's up to each individual to decide how to do their repairs.