Hi to All,
There are several different types of head cement made from a water base or a petroleum base. Some are epoxy and some are super glues. The thinner cements penetrate the threads better but can still be very hard. If you are using large threads or bulky materials you may want a thicker cement to fill in the gaps between the threads. Epoxy makes a very hard head and is used a lot in saltwater flies. Here is a link that shows
several of the different head cements that are available.
Here is a trick I learned in Alaska for making beautiful heads and they will never come un-done. Make a few wraps with thread and whip finish one or two times. Tye in a piece of acetone thread and finish building up your head to the shape you want and whip finish. Now add a drop of acetone to the head and all the the acetone thread will melt into a hard, shiny finish. You may have to hold the fly in you hand and move it around so the melted head stays right where you want it. I use to make black ants this way.