98:2 systems are available in different variants:
Open - means that the spray plastic (98 %) with the hardener (2 %) is also mixed spray in spray. The liquid hardener is sprayed by means of low pressure system into the spray plastic. The advantage here is that you can always hold on/stop with the machine without anything in the system can harden. The disadvantage in the poor visual check whether harder is injected.
In the closed system, color and hardener are combined in a mixing chamber. The advantage lies in an electronic monitoring, if really enough color and hardener come together. The disadvantage manifests itself in longer and frequent stops of the machine. In this case, the mixing chamber, hoses and nozzles may need to be freed (flushed out) more often from the activated material.