How long does Rustoleum take to fully cure?
DRY AND RECOAT TIMES
Allow more time at cooler temperatures. Dries tack free in 2-4 hours, to handle in 5-9 hours and fully dry in 24 hours.
Dries to the touch in 1 hour, to handle in 3 hours and is fully dry in 24 hours. Recoat at any time. Wipe off tip when finished. Clean up wet paint with xylene or mineral spirits.
Speed up any 9400 System project by adding Rust-Oleum High Performance 9400 System Accelerator. When mixed into the top coat, this special additive can cut drying time from overnight to just three or four hours.
On average, spray paint takes about 30 minutes to dry on a plastic surface and an additional 24 hours to fully cure.
Paint becomes sticky and tacky when it isn't able to dry thoroughly. Paint has trouble drying when the air is overly humid, or the weather is extremely hot or cold. Also, paint can have trouble drying if applied in thick coats.
For best results, apply 2 coats, allowing 1-2 minutes between coats. Remove any overspray from the auto finish immediately. Allow 72 hours for full cure before placing objects in the truck bed. Roll out coating evenly across the surface.
Allow treated surface to cure for 24 hours before exposure to moisture (rain, dew, sprinklers, etc). You may cover a treated surface with a tarp after the initial one hour cure to protect from rain and/or dew. Do not apply to hot surfaces or in direct sunlight as this may cause the surface to appear streaked.
Note: Sanding between coats is not necessary, but it will provide a better finish. After a coat has dried, use 220 or 240 grit sandpaper or extra fine steel wool to lightly sand surface. This will ensure good adhesion of the succeeding coat.
Waiting a full 48 hours between coats of spray paint will allow the first coat to cure — or nearly so, — before the second coat is applied, making the two coats of paint to dry and cure as two separate coats, capable of peeling separate from one another.
If there is too much water vapor in the air (humidity) or the room is warmer than average, it will take considerably longer for your paint to dry. To speed up dry time, close the windows and turn on the air conditioning or run a fan.
Does Rustoleum ever harden?
I think Rust Oleum will stay fairly soft for a while. Especially when thinned with mineral spirits. As long as it's not tacky it will harden as it cures. Might take a few weeks to a month for it to really get hard.
Light wet sanding with 600 grit sandpaper is recommended. The freshly coated surface can be buffed with wax or polish after one week.

For best results, allow to Primer to dry for 1 hour before dry sanding. Test in an inconspicuous area to be sure Primer is dry enough for sanding. Block sand the area using 320-400 grit sandpaper.
Note: Sanding between coats is not necessary, but it will provide a better finish. After a coat has dried, use 220 or 240 grit sandpaper or extra fine steel wool to lightly sand surface. This will ensure good adhesion of the succeeding coat.
Surface can be recoated within 1 hour or after 48 hours. If recoating after 48 hours, repeat the surface preparation steps. Light wet sanding with 600 grit sandpaper is recommended. The freshly coated surface can be buffed with wax or polish after one week.