Ironing 3D Prints Can Work, But Have Lots of Top Layers

This spot is the hottest on an iron of this make.

3D printed objects with large flat areas can have their looks improved by ironing the top surface, which is a feature in Cura.  This is done by running the hot end over the top of the print, to help smooth it into a smooth surface.

It is also possible to literally iron a print with a clothes iron, which I tried in a few different ways. It works, but there are some gotchas.  Here is a short list of my observations:

1. A medium setting on the iron is better than the hottest setting.  My iron was at about 80-90 degrees Celsius.  Too hot, and the plastic softens too quickly and you lose control.
2. The tip of the iron is where heat is concentrated. The middle and bottom are considerably cooler.
3. Use a sheet of parchment paper between the iron and the print, otherwise you risk plastic softening and sticking to the iron.
4. Move quickly, and inspect the results.  Treat it like spray painting, where many thin applications are better than one heavy one that may go overboard.
5. There is no need to "press down" on the iron. Just let gravity do the work. Concentrate on being even.
6. Make sure you have plenty of top layers in the print, and a good infill doesn't hurt either. 

The picture below was after ironing with three top layers. The thin top softened and thinned in the ironing process, and you can clearly see the infill pattern below it.  The surface is smoother (and shinier) but it also thinned and weakened, especially near the edges.

This is an example of a surface that was ironed smooth, but the top layers were too thin.  It's smooth all right, but it's also very thin and weak, and the infill pattern is showing through.