The Ion Painter seems innocuous enough at first glance, emitting a harmless low-power laser beam when the primary firing mode is engaged. Several seconds later, a multi-gigawatt orbital ion cannon fires on the target, neutralizing any combatants in the vicinity. The Ion Painter is a remote targeting device used to orient and fire the VAPOR Ion Cannon. The Ion Painter offers increased targeting accuracy via its telescopic sight, easily activated by the secondary fire mode of the weapon. Once the Ion Painter has been used to designate a target, it is highly recommended that the user put considerable distance between themselves and the weapon's area of effect. -- Official description

Not really a weapon, but still useful. The Ion Painter is a targeting gun which you use to paint a target. After the weapon has locked on, an Ion Cannon will send a beam of electricity to the target, pulverizing it. The Ion Cannon attack is similar to the attack a deployed Leviathan can unleash.

The primary mode is what you use to paint the target. You must hold it steady for a few seconds until the Ion Cannon acquires the lock. If you let go beforehand, you will have to start over.

The secondary mode brings up the scope, allowing you to target from a further distance. Just like the Sniper Rifle and Lightning Gun, you are vulnerable when zoomed in, so get the target acquired quickly, and switch to something to defend yourself.