You have to be careful when using less hardener, because if you don't use enough you will end up with sticky goo. It is better to use a slower hardener. Most products have alternate hardeners available. Same with paint.
Exactly right. To add just a bit, body filler depends on a chemical reaction to harden correctly. If you use too much OR too little hardener (catalyst) you change the base chemical properties of the filler. I still remember the day I learned that lesson. I added a lot more hardener to speed up the cure time and it actually slowed it down because I added too much. When it finally did set, it pealed off the panel when I tried to sand it. It isn't so touchy that you need an exact measurement of the amount of hardener but adding substantially more or less can backfire.
The hardening process is affected by heat. It will go off faster on a hot day than on a cold day. Experience will tell you how much to mix at a time on any given day.
