Watch some videos and read a few ways to do stippling on a polymer frame. My wife has a VERY NICE pyography machine (fancy wood burner).
After grinding the thick molded lines off the front and back strap, I put on a 3mm (1/8") tip and turned the burner on to 7. Final step was using a Dremel 404 Nylon brush for a light and simple polish of the surface to clean up the sharp edges without really affecting the overall pattern.
Here is the result of using a simple squiggle pattern as I dragged the hot tip across the polymer. I just took my time and stayed in the lines. WHole project was maybe an hour start to finish. You can also see the P380 Machined Slide stop I added and the Stainless Bedair Guide Rod.