Lets just say I was going to do some parts for myself. Being a pro, I have access to products that you guys don't unfortunately but you could pay a shop to do the same thing.
If the rails come in bare metal, hopefully the manufacturer oiled them down for transport to prevent rust. You do not want to paint over any rust. I dont believe in any refinishing product that claims to stop, convert, seal, trap, or prevent rust. Rustoleum , POR15, they are just paint. There is nothing special about them. All you need is air and moisture and you have rust. The only way to prevent air and moisture getting to the metal is a catalysed urethane clear coat system.
The bare steel should be cleaned thouroghly with a wax / grease remover. No soap and water. Mineral spirits or naptha is ok for starters if the part is really oily but I would follow up with a professional wax and grease remover. PPG , DuPont , RM, they all make a wax and grease remover. After it is clean, you do not want to get your greasy little fingerprints on the metal. I recommend using disposable rubber gloves.
So its now clean. The first product sprayed need to be a self - etching primer. Paint does not have etching properties meaning it will not stick to bare metal. Spray paint or even automotive basecoat or single-stage paint adheres thru a chemical adhesion, not thru a mechanical adhesion Meaning you cant just sand the bare metal and expect it to stick. The self-etching primer is basically a zinc-phosphate coating that actually bites into the metal. You can get it in spray can form for home use but I prefer the professional kind that of course, you probably cant buy.
After the etch, I would spray a primer-surfacer. This will fill any tiny imperfections and after sanding will provide a smooth surface in which to paint.
Next comes a primer - sealer. This fills any light sand scratches from sanding the primer - surfacer and also gives a uniformed color to achieve better coverage when applying your color coats.
Next comes the paint. You can use a nice, quality spray paint if you like. Its probably the only thing you can easily get to use in your garage. I like SEM brand black trim paint. It is designed to match oem black trim. It is expensive, about $12 a can.
Over the color coats should be applied a clear coat. In the perfect world, it would be a 2 part, catalysed urethane clear. The same thing that is on the exterior of your car. The modern clear systems are guaranteed for life against hazing, fading, peeling, cracking, crows feet, acid rain , etc. You can even add a flattening agent to kill the shine to a satin or even flat sheen. You at home will probably have to use a spray can clear. This is nowhere as good because it is not catalysed and therefore will not be waterproof and will not have the uv protection needed. But it may be your only option.