No argument from me on DIY and learning - I've had to redo finishes on my own various stuff with DuraCoat finding the perfect mixing formulas based on stupid stuff like humidity, or how DuraCoat reacts on various types of materials. Even one type of plastic may be more of a PITA than another plastic.
Good example is Magpul stuff, their polymer is awesome with DuraCoat, texture, absorbs and retains well. Then you take something like a cheap $10 butt stock that is more like ABS plastic, very smooth, bad starting surface for the finish to grip to. Cheap crap takes way more prep time.
If you are intent on wanting to engrave on firearms, you gotta start out smaller.
Practice on spare pieces of metal from a local hardware store, then progress to something like zippos, then maybe something bigger like a tool box.
Edit: Or even better, seek out junked gun slides and stuff. I've heard gunshows have vendors who sell old firearms frames/slides/parts for super cheap so people can practice gunsmithing on them.