I wasn't real worried about it adding strength, this project doesn't need it, just doing for visual effect. If I stain it first, then put in the splines, I will have to get the splines flush with the rest of it, and would probably upset the stain area around it. Usually this isn't a problem because if I had some walnut, don't need to stain it, just poly and the splines are already installed and a light color. The plastic was just a shot in the dark, wondering if anyone had tried it. Plastic will stay I could drill some tiny holes and the glue would travel from one side to the other and hold them in well enough, but I have also never sanded down plastic inlay'd in wood, so wasn't sure about that either...lol...kinda why I was asking if anyone had done that before. May have to come up with a different plan, maybe make the splines out of soft wood and go the other way, let them really take some stain and get darker...lol...they would stand out, but not as good, since I am not worried about them for strength the softwood will be ok.