Can you ad hardener to Rustoleum? It does need to be thinned out to be able to shoot.
Sorry Polcat, no Jack here that I see. Adam/Steve and Ddub, BUT HI!
Qualification, I don't use rustoleum so I have no experience with it. Not trying to be "smart" just saying it like it is.
I did a bit of checking though and found what I suspected. First off, there is MANY versions of Rustoleum. But what I found is that they seem to be either a "Acrylic Latex" (think house paint) or "Oil Based Enamel's".
Most likely you are using one of the "oil based enamels". What you would probably purchase hardener for would be "acrylic enamel", "acylic urethane" and "alkyd enamel" (could be also called implement paint). so, you can see that the chemistry is going to be different.
Now, with that said, I know Valspar makes a hardener for "all implement enamel" whether oil based, or synthetic enamel. So in a round about way, MAYBE. LOL Same advice to Adam, you are on your own, test it on a scrap piece first and make sure you are happy with the results.
I'll throw something out there though. I wouldn't want to spray oil through my paint gun...........
Now, if you have a "dedicated oil base enamel paint gun" it would probably be ok.
I will also tell you this, oil based enamels do not seem to take automotive paints above it, most often it seems to lift. The reducers in automotive paints are much harsher than what oil base can handle.
I hope I don't sound harsh or bashing, I am just trying to be upfront and informative.