Basically your compressor is undersized.
Ta!
but I says 6-25 so 6 x 1.5 = 9 soooooo
That is the bare minimum you'd need to sand etch glass or blast with a light blasting powder such as pumice or ground cherry pits. Steel shot or sand - which are what you need for cast iron, steel, etc - are much, much heavier requiring 20cfm or more AND at higher pressure, say 80cfm. Ergo you will need a compressor at the top end of that scale.
The compressor you have chosen is way too small on two grounds. Follow me and I'll try to show you the maths:-
Firstly 14 cfm is the
swept volume of the compressor. It is a THEORETICAL figure, not an actual one. For that you normally need to find out the FAD or Free Air Delivery of the compressor. FAD is the REAL, measured output from the compressor and industrial compressor manufacturers always quote this whilst the guys selling hobby and light (car) workshop compressors behave like estate agents when it comes to outputs - always quote the highest figure without explaining what they mean by it. As Norcon says this swept volums is in effect about 2/3 the theoretical figure, so your 14cfm unit is probably only giving 9 to 10 CFM FAD in the real world.
Next the receiver (air vessel) size is way too small for the application at only 50 litres. In order that you don't kill a compressor you need to run on something like a 1/3 to 1/2 duty cycle. In other words the compressor ideally should only run for 1/3 to 1/2 of the time you are operating the equipment on air. A 50 litre (or 1.76 cubic feet) tank drained at 14cfm will last about 8 seconds before the motor cuts in, at 6cfm the drain time will be around 18 seconds, always assuming that your air pressure at the receiver and the gun are the same. Even if you drop the air pressure at the gun to 60 psi, or half the compressor's capacity those figures will only be improved to 16 seconds and 36 seconds respectively. 60 psi probably won't be enough to pick-up a heavy abrasive so you'll turn the pressure at the gun up to compensate and the compressor motor will spend all it's time trying to keep up with demand. The result is that you'll run out of air repeatedly and have to wait all the time for the receiver to refill whilst the motor will run constantly leading to premature failure and in all probability very annoyed neighbours.
By way of comparison a 300 litre tank would give you a buffer of 48 and 108 seconds respectively at the higher pressure and double that at the lower pressure. A reasonable compromise might therefore would be in the 150 to 200 litre area, although you'd still require a 25 to 30cfm compressor unless you want the motor running all the time with the inevitable results.
Sizing compressors and receivers isn't rocket science, but it can't be done by pulling figures out of the air, either. Hopefully you understand my earlier comments
