Filling Holes and Building Up
Very thick plaster can be used both to fill in holes (much like Bondo) or to alter the gross shape of the cast. To fill holes, mix a small amount of plaster to the same consistency as what you used to fill the cast. Dab this mixture into the hole (try to avoid trapping air) until it is filled. When the plaster has dried sand the area smooth. It will be easier to smooth the area if the two plasters are the same consistency.
To buildup an area mix very thick plaster, otherwise it will run off or drip before it can harden.
For very smooth surface a slush coat of very thin plaster can be used. This is generally done using one of the smaller plastic containers. Put a small amount of water in the container (a few centimeters can coat an entire cast) and add plaster until the mixture is cloudy. Then apply this mixture to your pre-smoothed cast either with your finger tips, a strip of stockinette or a piece of foam. Spread out the slush so that only a very thin coat is applied. If you find that the plaster is drying too quickly try using cold water.
Sometimes you will not be fabricating a device on a plaster cast but rather a foam positive. Some second year projects, like the thoracolumbosacral preform shown below are examples of this. Since this foam cannot be cleaned up to a very smooth finish, you will need apply a top coat of plaster that can be smoothed. The slush coat method works quite well for this, although it may require a thicker application than if it were used on a plaster cast.
A TLSO foam positive with a thin coat of plaster that you may use in second year.
A close up of the exposed foam underneath the plaster slush coat.