Completing the painting of your steel entry door will not take much time, but it is important that ample time is given for drying. There are many factors that contribute to this, temperature, humidity, and ventilation, but ultimately the best resource is the directions on the label of the products that you will be using on the door. Manufacturers expend a lot of time and resources studying the products that they produce; the recommendations that you find on the labels are based on extensive research performed on the product. Read the labels of the products that you are using and follow the directions exactly. Ensure that the primer and paint that you purchase are meant to be used not only for interior and exterior use, but most importantly that they are also meant to be used on a metal surface.
Step 1 - Primer the Door
Now that you have the door prepped, you can begin the painting process. The first step is to primer the entire surface that will be painted. Primer provides an ideal surface for the paint to bond to, and will also give the truest color of the paint. It will also reduce the number of coats of paint that you will have to apply. The primer does not have to be flawless; usually one or two coats is sufficient the get adequate coverage. Pour some primer paint in a paint tray. Using a roller or brush, apply the primer in an even coat all over all surfaces that will be painted. If needed, you can use the smaller paintbrush to get to the crevices. Do not paint over the hinges of the door if it is still hung. Allow the primer to dry completely. While a primer can be dry in the course of a couple of hours, give it at least twenty four hours to dry thoroughly.
Step 2 - Paint the Door
Again, using the roller or brush, apply one coat of your paint color to the door. Allow the paint to dry, and reapply a second coat. Repeat this process until there is no primer showing through and the entire door is an even consistent color. You do not want the paint to dry on either the roller or the brush. One way to avoid this is to wrap the used applicator in a grocery bag and put it in the freezer. When you are ready for the next coat, simply take it out of the freezer and unwrap it.
Step 3 - Finish the Door
Clean all tools that were used in the application of the primer and paint. Paintbrushes should be washed carefully to ensure all the paint is removed. With paint rollers, simply dispose of the used roller applicator and wash the roller it's self. Once all the paint is dry and any needed touch-ups have been done, you can start to reassemble the door. Reinstall any hardware that was removed, and remove the masking tape.