I need to rattle can an old upper with Krylon. I thought I might bake it a little in an old toaster oven. How hot?

150-180.  You could try a heat gun also.

Thanks but no heat gun. I bought the toaster oven just for this kind of stuff so I appreciate the input.

+1

My results have been much better with an automotive heat gun.  I don't trust the accuracy of the thermometers on food grade ovens, and in my experience the smell can be a little overwhelming.

I'll also add:

Baking Krylon will NOT enhance the durability enough to warrant the activity.  (I've tested it).

Invest in Aervoe.  It is a much better rattle can product, and does benefit from heat curing.

^^^ Aervoe is good....even better when baked.  Yum.

Just be sure to preheat the oven and let it sit at temp for a bit before inserting the upper.
I've never done Krylon, but I wouldn't let it go above 200*

On a side note, I baked a handguard lastnight that I just cut down and touched up with Aerovoe. I started it around 225* and ramped up to 275* for the last 30 minutes.Total time was about 90 min. After baking, the paint had no smell left to it. That's when you know it is done.
I took it out and let it cool for about 5 minutes. Then realized the inside of the handguard was peppered with aluminum dust from the cutting.
Took it straight to the sink for a hot water scrub.
The freshly baked paint held up great.

I decided to try some Aervoe when Brownells was offering free shipping on anything. I have to admit it looks pretty darn good and only a few bucks more than Krylon.

I've never baked krylon (just never felt the need).

However, seems like I've read about folks using krylon on suppressors and the heat turning the paint a whitish/faded color.  I have no clue how hot a suppressor gets, so baking at a lower temperature may not cause that at all.

The only paint on a gun I've baked is when I used some flat black ceramic header paint to paint a FAL.  Turned out really nice and durable as hell.  It was a lot of work though, stripping the receiver/barrel of all the springs and cleaning it up prior to painting.  Baking at three stages of increasing heat took a little time.  It was worth it though.

Baking most rattle can finishes is just speeding up your cure; it won't have any effect on durability.  Now I do regularly pre-heat parts, even if just rattle-canning, but that is just to burn out any residual oils, and for better application.

Since you're really just dicking around, I'd say 175-200 would be plenty mon.  I'd pre-heat for about 10-15 min, and then post-bake for maybe 30 min.  That will set your cure.  

The only real advantage here is speeding up the process, which may/may not be important to you.  

Toaster ovens will work, but I'd back it up with a meat thermometer.  If you do any real heat cure finishes.

Seriously, a small airbrush and compressor aren't that expensive, when compared to the latest smart phones and other crap people just have to have.  If you really want to heat cure your finish, get the good stuff!