Yeah, what Smitty said. I would be asking myself how old is the stuff. Judging by what you said about the Neon. It sounds pretty old. The hardners need to be completely sealed or they start to gel up after a while.
I can give you a simple explanation of the products you found.
1. "Enamel Hardner": That is used with a single stage paint job. Single stage is the color, hardner, and reducer all mixed together and applied to the vehicle. The hardner is what gives the finish depth and a nice hard shine.
2. "Urethane activator: I believe that has the same function as the Enamel hardner only difference is the paint is a Urethane paint (more durable than enamel) Enamel Automotive paint is getting phased out and replaced with Urethane and Base coat/ clear coat.
3. "Epoxy Primer Catalyst": This also has the same function as the hardners. Only difference is, it is used with a 2 part primer (primer, catalyst). There is usually NO reducer mixed with this primer. It is great for high build applications. (filling minor imperfections before color coat). It is MUCH MUCH better than the old crappy Laquer primer.
Assuming any of this stuff is still any good, all you would really be using is the "Enamel hardner" and laquer thinner. You would need to get some "Medium Reducer" which is basically paint thinner only designed for automotive use and for medium temperatures.
I doubt you are going to prime the entire bus first. But if you were, you could use the Epoxy Primer.
Brand names for paints, hardners, reducers, activators are usually not mixed. Example: If you have a hardner made by Dupont and paint made by Omni you would not want to mix them because they might not be compatible with each other, resulting in flaking or peeling.
Its very possible that you could use the enamel hardner with the Rustoleum paint and have good results and last several years. But no guarantee.
The best place to find out a lot of info regarding this subject would be WWW.AUTOBODYSTORE.COM
. There is a wealth of info on the forums over there.
I would actually be interested in finding out if any of the hardners can be used with Rustoleum or equivilent. Automotive paint can run almost $100 a gallon. The hardener isn't cheap either but at least you could be saving a couple hundred bucks on the just the paint.
Keep us posted! GOOD LUCK!!