Robar does it in the same manner that people have been doing textured rifle stocks for a long long time and since they do it on their rifle stocks it transferred very well onto the Glock. Marine Tex, Acraglass, Devcon or even JB weld are some of the more common for an overlay texture or fill and shape jobs. Stippling has has gained more favor as it is easier DIY and is more durable long term. I tend to use Marine Tex for over lay type texture, fills, beaver tails, old style integrated magwells, prior to the heat formed one that I favor now. Or you could go with the sandpaper type finishes like ZEV tech used heavily was just an even layer of JB or similar on the grip and then embedding a silica grit in the tacky epoxy. Spray on be liner is also not a very good permanent solution. Just like the silica, the bed liner needs touch ups can be messy and feel sticky.