Short version, you need to break down the parts that are to be parkerized, either oxide blast if you are dealing with coated parts first, or final bead blast before the parts are parkerized if just old parkerizing/ bluing, then you into the actual parkerizing. Once the parts are parkerized, then you need to reassembly the parts back the assembly.
Take a simple AR bolt, and you have to pull is all the way down, including removing the ejector, before you can parkerize the pins and bolt body.
Truth is, since spray coating steel parts starts with the parts being parkerized, so the coating has something to bond to. Hence offering both Parkerizing and spray coatings services do go hand and hand.
Now the down side, if your smithing skill are not strong enough, then these need to be improved before you start offering the services.
Lets take a M9/FS 92 that someone brings to you to re-coat the slide. Before you can do anything with the slide, you need to strip it down completely. Once you have the parts stripped out the slide, then the slide and needed parts get oxide blasted to first remove the old coating and old parkerizing, the parts are then bead blasted to smooth the oxide cutting for a peen sheen that will be the base for the parkerizing, then parts parkerized, then needed parts coated, then the slide put back together.
And to take it one step further, in the case of a Garand or M1a, the gas cylinder on it will not take standard parkerizing. So what happens when you are refinish the rifle, the other parts are parkerized, and the gas cylinder outer surfaces are bead blasted, and then spray coated in a fashion to match the parkerizing in sheen and color.
To sum it up, the parkerizing of the parts is the easy part Prep and tank work), and the detail part is getting the parts first stripped down for parkerizing, then putting the parts back together without mar'g the hell out of the new parkerized finishes. Also, when I say parkerizing is easy, it really is not, since you need to work with both your magnesium and zinc parkerizing solutions to achieve the needed final tints if you are not just parkerizing all the parts to one color instead.
Bluntly, where most guys screw themselves, is not having a cost correct for the detail strip and reassembly of the parts before and after parkerizing, and either short change themselves, or bid the price way to high instead.