i did a plaster model once. it was very small though. i made a form out of basswood and covered all surfaces in vaseline. i used a standard art store plaster that the person in the store recommended.

in retrospect, i should have been more concerned with bubbles. ultimately they caused the whole thing to crack and i had to do it again. how to avoid that, i'm not sure.

for a perfectly smooth, almost glossy surface, use plexiglas for the form--it doesn't have to be a thick piece as that gets expensive--what i did was build a formwork out of plywood and face it with 1/16" plexi.
for bubbles--pour the plaster slowly, and knock gently on the sides of the form to get out the air.
i also know someone who used rubber molds for a more complex form so that the corners of the plaster wouldn't chip when coming out of the mold.
you might also want to look at ultracal, or doing a ultracal/plaster mix for more strength.
and if you have any thin areas, use wire mesh as reinforcement so it doesn't break.

One of my idiot studio-mates in 5th year thesis did her site model out of plaster. She cast it on her desk, then sanded it with a palm sander, thus covering the entire studio (everyone's models, drawings, desks, computers, etc.) with plaster dust. She soon realized that it was too heavy to move or carry, so she went down to the woodshop and built a cart for it that was the same height as her desk, then slid it off onto the cart...

Don't do it.

also to avoid bubbles, mix slowly and avoid stirring in air. add powder to water also to avoid trapping air.

i'd rather work with resin (much more toxic) than plaster as it's lighter and more resilient. you can also adj. the catalyst to cure much more quickly. but it'll kill your brain cells. sand and you can finish in a variety of ways.

remember though if you go the rubber mold route you will have to make a postive form, essentially making the site model first out of something like modeling clay (which actually does give you great artistic freedom on what it will look like) which will then need to be sealed with a resin or polyurethane coating prior to applying the rubber mold for the negative formwork....but unless its going to get a lot of repetitive use this is a lot of time and money for a site model for christ's sake, time and money which could be devolted to the project itself.

you can buy kinds of plaster at art stores which will advertise that they can be model-able simply by adding less water (making them more putty-like than runny, and even plaster of paris will do this)...but i don't think you will be happy with the results of this. it is a very cumbersome material to work with with your hands or tools and is better suited for filling an formwork in my experience.

Less is more and simpler is better when it comes to site models...but that's just me..save the experimentation for the design...but if you do go this route...PRACTICE AND TEST EVERYTHING FIRST!

sheet metal or flashing also works really well as a form liner. make sure to vibrate the sides of the form just after pouring to release any air bubbles.

if you can allow yourself to patient enough to allow the thing to dry completely, it sands beautifully. also, if you are making a basswood model to put into this site model, make sure the plaster is absolutely dry before letting the basswood touch it, otherwise you'll end up with the curviest basswood you've ever seen - maybe that's how gehry does it.

if you side model is thick, you may want to put some foam in the thick areas to reduce some weight

I am not sure what you mean by site plaster model, but the "traditional cake method" is layering foam panels, sand the edges, cover with plaster, dry, paint using artist's oil paint (greens and ocres) and lots of thinner.

i've made plaster 'sheets' in reusable plexi forms that are easy to pop-out and then you can work them just like wood, ie. cutting, sanding and glueing togther with a plaster mixture.
i've also made mdf forms that were disposable and used spray PAM as a release agent.
If possible - try to reinforce the plaster. I've had really good success with insect screen as a scale equivalent of WWF.

I had an art professor tell me one time that in sculpture programs they spend about two weeks teaching you how to mix plaster properly.

what I remember: the best way to mix without introducing any air is to make a fist and stick out your pinky and thumb (the surfer thing) and twist your wrist back and forth quickly, so you're vibrating and mixing the plaster at the same time.

i have had some luck with plastered models