One of the things that makes the P938 so controllable, despite its diminutive grip size and 9mm chambering, is the aggressive stippling on the front and back straps. When you shoot it, the stippling keeps it from shifting in the hand so it comes back onto target faster with each shot. The grips on the side are simple rubber and aid a little, but it's those front and rear surfaces that really make the difference.
The stippling on the Gen 3 Glocks is not very aggressive, and I always have problems with them slipping around a bit when I shoot them. In 90-degree, 90% humidity weather, it's even worse. After running a cross some YouTube videos on stippling Glocks, I decided to give it a try.
The really hardcore home workshop craftsmen who provided examples of their work turned out final products that are much prettier than what I did, and they were much more extensive with their approach. They actually smoothed the original stippling flat before going to work and stippled every part of the front, rear, and sides of the grips, as well as the frames. It was impressive and surely would provide a solid grip on the pistol. Unfortunately, all of that extra stippling could make a pistol hang up when drawn from concealment, and it would feel really rough against the skin after being carried for a couple of hours. I decided just to stick with the front and back straps as Sig did with the P938.
I used a simple wood burning tool I picked up for about $16 at a hobby store to do the work. The existing stippling on the front and back straps is a bunch of raised squares. I theorized that if I poked each with a pointed wood burning tip that the squares would end up being more aggressive in texture. I was right. I didn't practice on scrap plastic before going to work, so it isn't pretty, but, damn, it sure improved the grip.
I also used a rat tail file to deepen the undercut at the rear of the bottom of the trigger guard. I might make it deeper at some point in the future, but for now, the difference in the grip I can get is noticeable.
I finally tested it at the range, today. The difference in my shooting was tremendous. I had a lane at the range with three steel plates, which I arrayed from distances varying between about 15 yards and 25 yards. I did controlled pairs on each plate, transitioning from one to the next until the magazine ran dry. The way the pistol sights very consistently came back down onto the plate after each shot was amazing. Before the stippling, the pistol would have slipped around as my hands got sweatier, causing the sights to come back down more randomly and my grip to start moving lower away from the top of the back strap.
It really didn't take much stippling to improve the grip. If anyone has ever entertained thoughts of stippling their own pistols, I would highly recommend it. You'll wonder why you never did it before.
If you don't care about looks, you can go right at it. If you want something prettier, though, I would recommend that you find a piece of polymer to practice on, first, so that you can have a more refined look to the final product.