Sealing rusted spots for safety
If you step on a dirty rusty nail in a barn, the problem is the puncture wound and the dirt, not the rust.  Rust is just oxidized iron and not especially dangerous on contact. 
As long as it's clean with no sharp edges, burrs, etc., there is no need to seal it for safety.  

Sealing it may slow the rust from spreading though.  Clear nail polish would probably work as well as anything.