That certainly doesn't look like any keying hammer that I have ever seen, and the shape makes it fundamentally unsuited to that task. A keying hammer has a long thin end in order to fit between the rail and chair, that headwould not fit. Having said that, I am sure that I have seen just such a hammer fairly recently but I can't remember where or when.
Back on the subject of "sand blasting" please remember that it is both illegal and very foolish to blast with sand or any other medium containing more than 1% free silica. This is due to the high risk of contracting silicosis, a truly awful debilitating illness with no cure. It is prohibited under the COSHH Regulations, 1999. You must use proper, intended-for-the-purpose blast media. It is also highly advisable (essential in the workplace) to use an air-fed full face or full head mask or shield.
Remember also that the paint you are blasting into airborne dust may contain lead and other evil health hazards.
If you can get hold of one, a wet blast machine will give better results, is more versatile, much cleaner, and almost dust free (and therefore healthier) than a dry blaster. The downside is that typically a wet blast machine is around ten times the price of a dry blast machine.
Blasting is a great timesaver and really useful technique, but you want to live long enough to enjoy the results of your endeavours.