practice painting

I don't think you will get the same results from the plaster because it is softer and far more porous than plastic. What I would try is to apply two or three coats of gloss enamel paint (I would use a light color  (probably white), letting each coat dry a minimum of 24 hrs between coats.)  Then apply the final coats of the paints of your choice.

Depends on what you are trying practice. Are you practicing on how the texture of the model will look? Smoothness of application?

Or are you practicing how to use the airbrush itself? Line thinkness, coverage, free hand camo patterns?

If its the first, then the plaster will not do much good. But, if you are just practicing how to use the airbrush then it would make a decent medium in my opinion.

Agree - plaster is not a good subsitute for plastic.

Honestly, it would be well worth your while to just go buy the cheapest kit you can find and use that.  I have an old Tamiya 1/48 P-51D that I use to test out paint viscosity and try out camo patterns, etc before ever getting near my current project.

 I have also used old wings from a 1/48 B-17 (talk about having a lot of room to work!!).  Dont forget, you can paint the insides too!.

I had not used Metalizer paints in ages, so did a lot of practicing with my dummy parts.  SO much less stressful to practice on that. 

You can then sand off the paint, primer over it, etc. to get ready for the next time. I also find it very helpful if I want to try out different effects, or even decals without risking my current build.

Not to mention, spraying those parts after cleaning out the airbrush to make sure everything is good.

Good luck!

Chris

I use plastic knives, forks, and spoons to practice on.  I save up all the plastic silverware that come with take-out food, or you can buy a box of them for a couple dollars at a store. 

that is such a good idea steve . another one to put in my little black book

That is one of the reasons for primer.  Although primer is often used to aid paint adhesion, another good reason is to prepare the surface to be painted so that it represents a standard that makes given paints look the same.  If you use the right primer, paint will always look the same regardless of the surface it is going down on.  You may need to sand or do other preparation after priming, but the objective is to obtain a base that you can use your normal painting techniques over.  By the way, many good primers will work fine on plaster and obtain that good base for paint.