Primer is just that, it primes the surface so that you can apply a color coat and have it adhere to the surface. Primer is generally porous and if left without a top coat, rust will just return. If the bikes in question are just utility pieces that you are not very concerned about, maybe pick up a can of Rust-Oleum in a color that is close and finish the job. Who knows, with a bit of sanding here and there, you could maybe turn your bike into camouflage or make it look like a cow bike.