Richard,
As Jim says, your time will fly by. 20 minutes is hardly enough time to cover any one of the broad subjects that you've listed, much less all three. You might see if you can "negotiate" with your friend to see if you can get more time - maybe an hour?
I would also suggest that for each "segment" of your demo, you plan out the sequence of the tasks / techniques that you want to show and prepare a head of time examples of parts or figures at that stage of the build. Have some notes for yourself that you can refer to if you get off track.
So, for example, for basic assembly you might show how to cut parts off the sprue, then you might show how your trim and file the mold seams from the parts, but instead of taking 5-minutes to clean up some particular part, you just briefly show how you use your X-acto knfe first followed up by a needle file and sand paper (or what ever your personal sequence is...).
You can then take a couple of parts that are already cleaned up and show how you glue them together. Show how you dry-fit first to be sure of a good fit and how you apply the glue. But only glue two parts together rather than try to assemble an entire figure.
You can then take a completely assembled figure and explain how you undercoat with Chaos Black spray paint, maybe show the figure taped down to a piece of cardboard and hold up the spray can. Maybe spray a single light coat on only one side to show how you use light, multiple coats rather than a single heavy one, but then stop there.
(BTW: In so far as airbrushing, maybe just hold up an airbrush and tell the audience what it is and how it's useful, but don't spend too much time. Any model builder ready to move to an airbrush is already past an "introductory" demo at the level I think you're aiming for. Consider that for a later, more "advanced" seminar.)
You then move to a figure already base coated with black to show how you block in the basic colors by brush. Explain how you thin the paint, how you clean the brushes, etc, but only briefly demonstrate any actual brush painting.
You then move to another already undercoated figure with the basic colors painted on and show how you apply inks or washes, but again only briefly demonstrate any actual painting. Maybe just put the ink on a single arm or leg.
Next would be a figure base coated, washed/inked, and ready for, say, dry-brushing. You'd then explain what dry-brushing is, show how you get the paint brush ready and remove most of the paint, and then, again, only briefly demonstrate on this figure how to actually dry-brush - maybe again a single arm or leg.
Next would be an example figure already dry-brushed and ready for detail painting. Once again, you show the brushes that you use to detail paint, share some tips maybe on lighting, holding the figure, holding the brush, paint thinning, etc, and only briefly actually paint some detail part.
Finally, you would move to a completely detailed figure and show how you base it with PVA and fine sand and static grass. If you already have a small cup of basing sand, you can quickly put a dollop of Elmer's glue on the base, spread it around with a brush and then dip the base into the sand. Have all of this laid out and ready to use before you start the demo.
If you're doing vehicles, the process would be similar. But at $40 a pop for a Leaman Russ, your examples might have to be sub-assemblies rather than entirely new vehicles until you get to the finishing and painting. You might then have a "Harvey Two-Face" example with some areas still in the raw and others at successively more completed finishing steps.
The key to saving time during the demo is have many examples already prepared showing each step and have all of your tools, paints, ect, ready in the sequence that you want to use them. If your space is limited, you can put all of the stuff for each example/step into a small box and bring that up as you move from each step to the next. You can also pass your examples around to the audience (if it's a small, round-table type demo) so that every one can get a close look at what you've just described.
You might also think about making the title of you demo an "introduction" so that attendees don't have too high expectations, especially if you can't get much more time.
You might also be able to parley your "introduction" into a regular series of workshops covering the various major subject areas in more detail.
Our AMPS club usually has some sort of demo or round-table technique/tool discussion each month, and because of store hours, we have to limit these to only about 30-minutes. You'd be surprised at how fast that time goes by if someone actually spends it doing the modeling work rather than just briefly showing the technique and moving on to the next step.
Even then, things like painting can take a lot of time to show because there are so many steps which use different techniques. Our recent figure painting demos were broken down into 3-parts over three meetings, and we still could have spent twice as much time.
Preparation and a good plan along with examples finished to each step/stage of your process can really maximize the time you do have, though.
Good luck and have fun!
