Nitrogen in tires is not a gimick. Aircraft tires use nitrogen as it is much more stable with temperature changes and pressure changes(altitude) If you filled an aircraft tire to about 150 PSI (some tires are inflated to over 200 psi) with normal air on the ground it could easily double at altitude and blow the tire up with catastropic results. on landing the heat generatred by landing friction and brake application is so high you get the same expansion problem. Nitrogen pressure in a hot tire is not much higher than a cold tire. Check your car tire in the morning, then after an hors drive in the summmer and you could see 5 psi difference easily. Race car tires need to be kept stable as they are balanced with tire pressure and the heat generated is alot more intense than in a passenger car.
I fail to see the benefit of using nitrogen in a painting application, the cost of a contract or purchase of a bottle can't be that much of a savings over a decent compressor. Plus it would be dangerous to displace all your breathing air with an inert gas. i can speak to that firsthand as we had an issue at work releasing nitrogen into a small room. luckily headashes were the worst ingury but it could have been much worse.