I’m thinking about getting a airbrush unit for doing tombstones and a few other items. Up to this point I have been using tinted Drilock and washing a paint over that. The problem is all the stones basically look the same. Any good techniques or tips of using a airbrush to add some character to tombstones ?
Airbrushes are great for certain techniques, but The Skeleton Crew is right, for the most part they're not very effective when it comes to traditional tombstones. Look at a picture of the Granary Burying Grounds, and you'll notice that that problem you seem to think you have of all your tombstones looking alike is pretty much what you get if you're going for ancient graveyard chic.
Older graves tend to look the same. But if you want to shake it up, you might find uses for your brush that step outside the normal gravestone boundries. I've used them in the past to attempt the look of sandstone. This year, that tombstone is getting a makeover because while it's common to see sandstone markers in the Southwest, few up here in Oregon have any clue why the tombstone looks the way it does.
Another chore where it comes in very handy is darkening the lettering on a tombstone. You can find a YouTube video that shows how an airbrush can be used instead of a paint brush at https://youtu.be/y6FzoJ8HsEE?t=552
The airbrushing begins at roughly minute nine.
It can be used to shade in complex detail work and save hours of painting time. Still, you'll notice that just about everyone out there making tombstones uses techniques that don't involve airbrushes to create their tombstones' overall look.
That said, Just because it isn't being used to create a unique look, doesn't mean it can't be. You may be the one to show off your technique here on the forum in the years to come. I've begun experimenting with my airbrush to create shadows that I want to appear regardless of actual lighting. You may find a new approach that others will be using in the future.
That's the way the forum works. Folks show us what they do, we try to repeat it while adding our own spin to it. Then we share what we did with others. Don't be afraid to try anything you think might make your tombstone look better. I think any of us who make them will be happy to tell you that it's pretty easy to paint over any mistake you might make, and brag about the happy accidents we find along the way.