A very clean surface is necessary. Use a couple of different solvents. Brake cleaner, acetone, denatured alcohol are some I have used. Dawn dish washing detergent is a good final clean.
As said above the surfaces must match. I am always surprised at the low polish on very nicely blued guns. A 1500 grit finish will almost not take the bluing. Some where between 240 grit and 400 grit is about right. But it has to match the rest of the gun. Polish in one direction, so all the “scratches” line up.

I have use Vans. It is more blue than black.