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Working With Polystyrene Parts and Kits

Working With Polystyrene Parts and Kits

  by Andy Slater

The stuff we're talking about here is the stuff that injection moulded plastic kits are made from i.e. not the 'fluffy' white stuff that is used to make packing materials (although that does have it's applications in modelling). Kits made from this material, and they do tend to be whole kits because if you can justify the cost of the tooling for one part of the kit you can probably justify the cost for the entire kit, tend to have a number of similar features/problems.

  1. Lots of parts all of which are attached to sprue and some of which are quite small
  2. A number of components of the model that are supplied as two halves and which will need to be joined and therefore have seams to deal with.

The first thing to do with any styrene kit is to carefully read the assembly instructions... before you move on to totally ignoring them. Seriously though, the best method of assembling a kit will vary depending on whether you want to achieve 'novice' or 'expert' standard results. For example it is 'usual' to assemble and then paint a model however some pieces will be easier to paint before assembling. The instructions suggest one assembly sequence. It's up to YOU to determine the BEST method.

Washing

Styrene parts will have residues left upon them from the moulding process and although this will probably not affect the assembly it will, in combination with oil (from fingers) and dust (from everywhere!!!) make a mess of your paint job. Rather than suggesting at each stage of assembly that it may be prudent to wash the parts in warm soapy water, rinse them, and air dry them, I have chosen to mention it here and leave you to decide when, and how many times, it is best to wash parts.

Removing Parts from the Sprue

I've seen various methods suggested but my preference is to use sharp wire cutters to snip though the sprue fairly close to the part but not right up against it. When the part has been completely isolated from the sprue framework use a sharp knife to cut away the remaining small pieces of sprue.

Joining Parts

Parts that are moulded in two halves, such as the bodies of aircraft, should be joined before painting (although the cockpit interiors must of course be painted beforehand). Polystyrene cement is the best glue for the job as this 'welds' the plastic parts together by 'melting' the plastic. Liquid or tube type cement can be used and probably both should be used depending on the application.

Tube type cement is good where a strong bond is required and where there is not too much chance of excess glue coming into contact with the outside of the model. If you use too much it will ooze out of the joint and mark the surface (remember that it melts the plastic in order to join it). It also has a tendency to be 'stringy' when applied and these strings can also be a nuisance. Possibly the best way to use it is to apply it with a cocktail stick rather than trying to apply it straight from the tube.

Liquid cement is generally more controllable. It works in the same way as tube type cement i.e. it melts the plastic in order to make the bond, but it is much more liquid and can be applied with a fine brush and with no risk of stringing. The usual method is to hold the parts together such that there is a fine gap between them. The liquid cement is then applied with a fine brush allowing capillary action to draw it into and along the join. The parts are then held firmly together until the join sets.

Which ever type of styrene glue is used, an essential point to remember is that should you apply so much glue that it oozes out of the joint, DON'T TOUCH IT. The damage caused by glue that has oozed out and is left alone will be less than the damage caused if you attempt to wipe it off. Let it dry, and then go about patching it up.

For parts that have been painted and which are being attached in the final stages of assembly you might like to consider using white glue. This will not give as strong a bond as the styrene glues but neither will it mar paintwork or parts. Clear plastic parts are susceptible to damaged from styrene cement and are probably best attached with white glue.

Using latex to attach parts that are likely to be damaged at a later date is also worth considering. Depending on what you do with your models, some parts are more likely than others to get 'broken off'. If these parts are attached with latex then such 'accidents' are more likely to result in the part being completely detached in which case it is no big deal to reattach it.

Another issue with styrene kits is that it is usual for alignment pegs to be moulded onto the parts. Some modellers suggest that these should be sanded off as a matter of course as they can hinder correct alignment more than they assist it. The author is inclined to assess this on a joint by joint basis i.e. if the dry fit is good with the pegs in place I leave them alone but if they get in the way... adios pegs!

Filling, Sanding and Priming

Possibly the biggest downside with styrene kits is that all of the 'solid bodies' are composed of two halves which are joined together resulting in seams that have to be hidden by filling and sanding.

There are pretty much always some seams, sink holes (small depressions in the surface left over from the moulding process), and marks from where the part was attached to the sprue that require filling. Gaps and holes that are big enough can be filled with a dab of modelling putty applied using a cocktail stick. This should be left slightly proud and sanded off later.

For fine gaps and seams it is probably best to take advantage of capillary action to do the filling. Mix one part of white glue and three parts of water with the tiniest of tiny amounts of washing up liquid (to break the surface tension) and apply this to the seam with a fine brush. Capillary action will draw it into the seam and now that it is wet you can apply neat white glue with the brush to fill the seam.

When the filler is dry, any obvious excess should be sanded away with a fine grade paper.

The next step is to prime the model with a light coloured primer and, when it's dry, examine it under bright light. The even colour of the primer will probably show up all kinds of surface imperfections and faults in your filling/sanding. Further rounds of filling, sanding and priming should follow in order to achieve a surface that is ready for painting.

Painting

By now the main parts of the model will have been primed although it may be more convenient to paint small parts before they are detached from the sprue.

The decision of whether or not to remove parts from the sprue will vary according to the part. In either case you will need to clean off any flashing and/or seam lines with a sharp knife and/or fine sandpaper. Leaving the part attached to the sprue will also probably result in some touching up being required when it is finally attached to the model. An alternative is to detach the parts, clean them up and then attach them to some temporary structure (pins or cocktail sticks for example) to make them easier to handle for painting.

Styrene parts are traditionally painted with enamel paints but acrylic can also be used.

This article is copyright (C) Andy Slater and is used here with permission
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