an organic vapor cartridge respirator will work with iso's IF used IAW the MFR spec. BUUUUTTT..it is very difficult to use them that way. The problem with the CTG respirators is "breakthrough" or when the cartridge becomes saturated and no longer will remove the contaminate in question. To calculate the time to breakthrough you would need an atmospheric monitoring program to figure out how long your CTG would last given the conditions where you are spraying. This is why most pro shops use SAR because it is way cheaper than all that other overhead trying to use a ctg respirator. Also keep this in mind. If you used a o-vapor ctg respirator for 10 hours and changed ctgs every 1/2 hour you would pay for a Hobby type SAR system in ctg costs alone.
hope this helps to explain