Not sure why you want to paint it, hard chrome is a good finish and very durable to wear and used to be the ultimate finish.
I used to blast the sides with the top of the slide a little coarser than the sides to be non-reflective before chrome. That worked well...Then hit the top sight round with low heat muffler black ceramic paint baked on. The sights were already matte black. No paint really sticks to chrome, but the sight plane is not a wear surface so I never had any scratches.
I wouldn't be sanding on that chrome finish, as it could really mess up the appearance of the chrome...then after the paint attempts fail you will have a mess on your hands. It is VERY difficult to get the chrome off for any other kind of finish as the chrome penetrates into the metal.Usually, once hard chromed that is the finish you have. But it is a great finish.
You could try very lightly abrasive blasting and use an epoxy primer or a newer type but just accept it will have to be touched up. The sides and the frame I left in satin chrome.
I'd recommend just to matte black the slide round top with flat paint and leave it there.
