Spray paints are finicky when it comes to temperature.
Flip a rattle can over to the directions and look at the optimal weather conditions and do the math: spray paints want to be at about 70 degrees Fahrenheit and low humidity. So what do you do when you still have to spray something outside without these optimal conditions?
The trick is to match the paint temperature to the surface temperature as best you can. If the paint is warmer than the surface or vice-versa you will likely get poor results. Usually if one is warm and the other is cool you will get a wrinkled surface finish to the paint as it dries. Unless this is the desired effect, try to get them both the same. The idea here is to trick the paint into it feeling like these conditions even for just short bursts at a time. This technique works better on smaller projects. If you have a larger task, you may need to wait for better conditions.
What I’ve found best is to use a container of hot water (just under 120 degrees Fahrenheit) in a container that closely is the volume of the can itself. You can likely microwave a few cups of water and pour it into the container with the rattle can already in and set some weight on top of the cap so it doesn’t float away and keep that submerged up to the cap. If you have a tea kettle to heat the water, even better. This is not a substitute for shaking the can and mixing the paint so make sure you do this as well. Shake the can for 2 minutes and place it in the container, add the hot water and weight then move onto the surface prep.
Next is surface temperature. The easiest way to achieve this is likely your hairdryer. Of course you still need to make sure your surface is clean and ready to paint so I recommend washing your hands well and cleaning the surface with alcohol. Now that everything is clean use the hairdryer like you might on your head: not too close and keep the dryer moving! Depending on what your painting a hairdryer might be enough to melt your project. Occasionally run the dryer past the back of your hand or inside of your arm to see if it is indeed getting warm. Try not to keep touching your parts to check the surface temperature.
Now for your location: if possible try not to be spraying while it’s raining or snowing! Those drops will mess up a paint job too. Ideally the wind is also not whipping either. If either of these are happening, hold off until conditions improve. If you have a garage you can pop the door open and spray out from the safety of cover, spraying out of the door. If no structure is available to paint from, just wait to the last possible second to bring your project outside so it doesn’t cool off too much.
Work in short bursts: When spraying outside its best not to hose down your project. Do multiple coats over multiple trips outside. As always keep you proper distance from nozzle to your work (12-18”) Between coats you want to use that hairdryer again to set the paint and gas off the solvents in the can that help it lay down. These solvents are often are what cause paint to drip and run. So the key is dusting on the paint, dry the part the best you can and start the next coat with minutes of each other. Don’t forget to shake the can and place it back in the hot water while blow drying the project. If you need to heat up more water between coats, do that as well. This is like being a short order cook where you are always doing something and not waiting like you might on a warm, sunny day.
Once the final coat is on, leave it alone. Don’t try to make the surface hot again, the paint needs to chemically cure over time. Many mistakes happen at the end of a project where you touch it too soon. My recommendation is to leave it alone to gas off for 24 hours after the last coat is on. If you can leave it in a garage or shed, that’s better than leaving it outside. If you can bring it inside without turning your home into a huff house from fumes, even better!
I hope this helps someone out as we move into winter temperatures but want to keep working on projects. Feel free to add anything you pro painters have to painting in sub optimal weather.