Preparing the Castings

Introduction

Like the laser kits we experienced last season, technology has allowed the cottage industry to produce low run high quality kits. With the advent of readily available RTV Rubber and high quality plasters, the home hobbiest can now venture into mass producing their models with minimal capital investment.
Of course, it isn't all that simple. After selecting an attractive subject, the manufacturer has to design a model in pieces that can be readily assembled and cast in plaster. This aspect becomes more challenging when the subject has multiple sides, angles and roof lines.
Thus what makes a plaster kit exceptional is a reflection of the time and effort that was invested in designing the model and making the masters followed by how well the castings are made. All the earlier work may be for not if the castings are full of bubbles and flash. So be wary when you purchase a kit to ensure the product quality meets your expectations. Fortuntely though, most manufactures take emmense pride in their work and imperfections are very rare.

One of the attractions of plaster is it's ability to capture almost any surface in great detail. Plaster can be used to replicate almost any type of surface: wood, stone, brick, etc And as you will notice, additionals details can be cast into the model such as lintels, brackets, NBW, conduit, etc.

One addtional feature I like about plaster castings is the speed of assembly. Since most of the model details can be cast in the surface, there is less that we have to add later. Thus the model goes together quickly and with ease.

Handling

Plaster castings are typically made from high desnity fine grained plaster such as dental stone or hydrocal. Though this material is very strong, it can break quite easily if you are not careful. Even though the thicker the casting, the more tolerant it will be from rough handling, it can still break. Therefore a few points to keep in mind when working with plaster castings:

Assembly Sequence

No Surprising, the assembly sequence for a plaster model is very similar to what we followed with the laser structures. Here is the suggested sequence we will be following:

Removing The Packaging

Due to the fragile nature of the plaster castings, the manufactures have to pay particular attention to how they package their product as it must be able to withstand shipment and possible (rough) handling by the Post Office.
So before you rip open the packaging to get at the castings, lets take few minutes to review the various packaging methods you may encounter and how best to remove the castings without breaking them.

Watch out for broken parts loose in the packing material!


Between pads of foam

Vacuum wrapped on cardboard - carefully cut around the edges. Donít leverage the plaster part Ė It will break!

Wrapped in padded material / peanuts, etc.

I've even seen a kit shipped from overseas where the castings were hot glued to white styrofoam. Unfortunately there was no "give" to this arrangement so the fragile castings were all broken in many places by the time they arrived!

Casting Preparation

Before we can launch into the assembly and painting activities, we must first prepare each of the pieces. Like all casting methods be it resin, plastic or plaster, there are mould lines & imperfections that require attention to ensure the finished model is not compromised.

After you have carefully unwrapped the castings, inspect each one for imperfections such as voids, flash or bubbles, Each and all of these must be addressed before proceeding,

Flash - occassionaly, there is some spill over of the plaster during the casting process that should be removed which is easily done in a number of ways. Using a sharp modeling knife, scrap off the offending materia. You could also use sand paper or even a file. Just be careful while you are doing this so that you don't remove more than necessary or break the casting.
Voids - appear when the paster has not filled the mould completely leaving gaps in the casting. Depending on the size, these can be filled in with a little baking power and super glue or with a loose mix of plaster.
Bubbles - are a result of holes in the rubber mould that the plaster captures in the casting. This can usually be removed with careful scraping with the hobby knife.

Carving a Casting

On the odd occassion, you may have a need to carve teh back side of the casting. The casting process involves pouring the plaster into a five sided rubber mould (4 sides and one face side) as the sixth side is left open for the plaster to be applied.

As shown in the adjacent photo, the back side of the crumbled stone walls are exposed so we needed to carve some stones into the surface as a flat layer of plaster is not very prototypical. To do this, apply a few drops of clean water to the surface, allow it to soak in then using a sharp instrument, carefull carve out your stones. This goes quickly and is quite enjoyable to do.
Windows & Doors

Invariably, the wall will have windows, doors or other types of openings that need to b ecleaned out. The manufactures often leave a thin layer of plaster in the openings to provide extra strength to the part unitl it is assembled. As with carving the back side of the castings, these are easy to clean out.


Step 1 Wet the casting area with a few drops of water.

Step 2 With your knife, cut a hole in the centre of the opening and slowly carve the edges away. Add more water if the casting starts to dry out.

Step 3 When you approach the edge of the opening, take small steps and slowly make the opening large enough to accommodate the window/door casting. e.g. sneak up on the snug fit.

Minimizing Gaps

Believe it or not, there will come a point in time where you will ready to start assembling the model - or so you thought!

Before we start gluing the pieces together, we need to ensure the adjoining surface meet perfectly - with no gaps. Remember what we said earlier, the fit of the plaster pieces is very unforgiving - you can not force the pieces into other shapes other than what they already are.

The secret process / approach is just careful sanding.

In the next photo, we have two wall panels that are to be joined end-on-end to make a longer wall section. We took both panels and laid them together and noted some slight variation between the matting surfaces. A sheet of 150 grit sand paper was then placed grit side up on a flat surface. We then took the first plaster panel and held it perfectly vertical to the sand paper with the adjoining wall edge face down on the sand paper. Next we carefully moved the wall panel back and forth across the sand paper. The idea is to straighten out the edge of the wall. Do this slowly and carefully so that you don't remove too much or create a curve through wiggling the piece.
Repeat this proceess for the second wall panel and don't forget to check the fit between the pieces very often. You only want to remove enough plaster so they fit tightly together.

The adjoining edges of the castings have been sanded so the pieces fit tightly together.

The same approach is followed with joining two corner pieces.

The photo on the right demonstations the scenario where the edge of one piece butts up against the rear of a second. Once again the key objective is to removed any undulations between the mating surfaces to ensrue a tight fit. As described above, sand the edge fo the iright piece so the edge if perfectly square and straight.

To sand the back of the casting smooth, just rub the castings gently across the sand paper.
You may have to clear out the plaster dust off the sand paper as it will eventualy clog up the paper.

Here is an example where we have a number of walls coming together all at once. Each junction had to be carefull saned to ensure a tight fite when all the pieces came together. It is best to dry fit all the pieces before you start gluing them together.

With this kit, the joint between the side and end wall will be covered by an addtional pillaster that will be added. Of course you still need to sand the pieces to ensure a perfect fit between all the pieces.

45 Degree Corners

Some model kits are made with the adjacent walls meeting at a 45 degree angle. This is to allow the wall details to go right to the corner without interruptions from an vertical joint line.
Once again our objective to ensure the joint is tight with no gaps.
In this case, I made myself up a wooden block cut at 45 degrees. I then placed the wall casting on the wooden block and used the 45 degreee angle to guide my sandpaper that was attached to a sanding block. Sand the casting carefully and test against the mating piece ofter - you want to "creep" up on the joint.


Here are two walls the meet at a 45 degree angle.

Come assembly time, the first three corners went together with tight seams.

Despite my best efforts, I could not get the forth corner to come together tightly. I will now fill the gap with a little bit of plaster and carve in the brick details.

In summary, keep these points in mind:

Assembly - At last!

My glue of choice for plaster castings is White Glue. Since we are dealing with very porous surfaces, quick set glues a not ideal. A slow setting glue that works with porous surfaces is ideal. Yellow dries to fast as the plaster draws the moisture out of the glue.

Though you can obtain a good strong join with an adhesive, it's very advisable to also add wood bracing to provide that extra level of strength and security. Most kits include some extra wood stock for this very purpose.

Lets take a look as the two wall panels we prepared above.
In addition to applying a bead of glue along the mating surfaces, we added two square strips of wood across the back to add strength and to keep the walls panels straight and in line.
Notice we have laid them face down on a flat surface and glued the wood strips across the back.

I "carefully" placed a few food cans on top the assembly to ensure the joints were tight while the glue set. This was left over night to ensure the glue had completely dried before handling the pieces.

Here is the building with the 45 degree corners. In this case I have installed 1/4" x 1/4" wood stock in all the inside corners.

Another secret to good assembly is to clamp all joins while the glue dries. We used some weights (food cans) in the above example, you can also use rubber bands, bar clamps, etc. But remember, don't be too agressive with the assembly process. Plaster is fragile and will break if you force ii into a position it doesn't assume natually!
This is an excellent example where is used just a bit too much clamping pressure to close the last corner of the building and the end casting cracked diagonaly across it's surface - see the white hair line crack. I applied some wood bracing on rear of the wall to brace up the broken piece.

In summary, keep these points in mind when you are assembling your model:

This now concludes part preparation and basic assembly techniques.


This Web page is maintained by Grant Knowles and was last updated on Nov 9, 2009.