Here is another thought. If we treat chem like the buff hammer, we could get some interesting effects.
Here is a quick image. The blue bar on the left is fire resistance. Each bar section is +20% fire resistance, all the way up to 100% (5 bars). If the bar drained completely over the course of 100 seconds, you would lose 1% fire resistance per second. Or, if the gradations were hard (only updated every section), -20% every 20 seconds.
Chem (and other fire tools) could then operate in one of two ways. It could work like now, with a single click and a one second cooldown, giving 20% resistance per click. Or, it could have no cooldown and work independently from the repair tools. Aiming at a component and holding the button would gradually increase the bar, as well as slowly put out fires. More than one fire suppression tool could be used at the same time (which makes sense), but no other tools could be used, including buff. This would be like an active cooldown, since you are literally cooling down the component while using it.
I think fire suppression should work this way anyways, even with the extinguisher. Fire suppression would have two stats just like the repair tools. Suppression, meaning how many fires it will put out per second of use, and resistance, which would fill the resistance bar. Other tools could be created as well. Stats for tools would then look something like this.
Tool | Suppression | Resistance (stats are per second) |
Extinguisher | -10 fire | +5% |
Chem Spray | -1 fire | +20% |
Water bucket | -3 fire | +10% |
Wet blanket | -20 fire | +1% |
Moonshine (joke) | -0 fire | -50% |
The good thing about this mechanic would be that any fire tool (depending on the stats) could be used to increase the fire resistance or put out fires. Some would just be better at one or the other. An 80-100% 'chem cycle' would still be relatively easy to maintain with chem spray, and perhaps possible with crazy extinguisher skills in smaller areas.