You can be the best painter on the face
of the earth, however no degree of skill will compensate for defects in the plastic surface underneath if they are not repaired prior to the finishing process. In this
multi part series I will discuss the different types of defects along
with the materials and techniques used to deal with them starting in
part one with common defects. Many of these defects are simply unavoidable as they stem from the molding process itself therefore will be a part of every build no matter what price you paid for it or who the rave manufacturer of the moment is, why that is takes a basic understanding of injection molding. A styrene injection mold consists of two aluminum or steel halves with an impression in each side for one half of the part to be produced. When the halves close a cavity is formed which liquid polystyrene plastic is pumped in to under hydraulic pressure which then cools, the mold halves are opened, and pins push the completed parts out preparing the mold for the next round. To see the molding process in further detail take a peek at the video below.
With all these splits, cavities, and pins molten plastic doesn't always behave as it should and will often seep in to these mechanical joints creating all manner of ridges and depressions or may cool too quickly and shrink causing dimples and warpage. Some are easily dealt with while others can be in complex spots near cast detail or a hard to reach crevice and may be extremely obvious or barely visible which brings us to defect number one...
Mold Lines
Mold lines get their name because they stem from molten plastic seeping between the two halves of the mold leaving a raised line down most commonly the center of the part. Most are light and easily scraped or sanded away with fine grit sandpaper, however some like those on a few vintage kits may be so thick they require a bit of carving with a #11 hobby knife blade followed by a good sanding with 220 grit to remove them. Fortunately on many kits they don't interfere with the cast in detail, however in the case of some pieces they may cut through raised bolts or sunken channels which can make them a pain to deal with and extend your time spent preparing for assembly greatly. The photo at right shows some in the center of a rounded bar highlighted in purple.
Ejector Pin Marks
As you saw in the video, when the molding process is complete a set of pins pushes the completed parts from the mold. These pins also called "ejector pins' don't always fit completely flush with the mold surface and as a result molten plastic may either seep in to the depression where the pins rest or flow around a pin that is stuck out further than it needs to be. The result is a raised or sunken circular impression known as an "ejector pin mark". Most manufacturers try to orient their pins in such a way that the marks will be on the back of a part or in an area that will be hidden once assembled, however there will always be some that need to be dealt with. Raised ones are the easiest to take care of since they can be simply lobbed off with a hobby knife and sanded flush. Sunken ones will need to be filled in with putty or gap filling superglue.
Flash
Flash is a result of a mold that didn't close properly allowing molten plastic to seep between the halves. Flash is sen as a thin semi transparent skin of plastic surrounding the part usually where a mold line would be and can be highly irritating depending on what type of part it's surrounding. The only way to repair flash is to carve it away carefully with a #11 blade in a hobby knife and remove the residual with sandpaper and files. Occasionally depending on the severity you may loose some detail such as bolt heads if the material is thick enough and passes through them. Though not near a part, the photo at right shows an example of flash on the sprue of a kit.
Sink Marks
Sink marks occur as a result of molten plastic not shrinking uniformly as it cools. The result is a dimple or groove and are commonly found on thicker pieces such as support beams and model car bodies, especially those with molded in headliners and dome lights such as AMT's 1971 Dodge Charger R/T. The repair of sink marks if fairly straightforward, simply build up a layer of filler material and sand it smooth so the dimple becomes level with the rest of the surface. In the photo to the right you can see an example that has been filled in with Squadron Green Putty.
Warping
Warping is another result of molten plastic cooling and is almost always found in large flat pieces such as modern sports car chassis plates and aircraft wings. Warps can be a trick or two to repair, if not done correctly you risk damaging the part or breaking it beyond repair. To repair a warped part you will need an old pot filled with water and a stove or microwave. Heat the water to just below boiling and dip the part in for several seconds using a pair of tongs. Pull the heated part from the water and gingerly (read don't be the Incredible Hulk) bend and flatten it to the shape required. Repeat this process working slowly until the warp is flattened using rulers or a table top to ensure the part is completely flat. Patience is the key here because if you bend it too forcefully or use too much heat you WILL damage your kit.
Seams
Seams are found where two pieces of the kit join together. Most often aircraft fuselages or auto engine blocks will be molded in two halves, however any number of subassemblies could present a seam. They may be very simple to repair, or extremely difficult depending on location, size, and surrounding detail. The photo to the right shows a whopper of a seam in the bottom of Revell's 1/48 Spitfire Mk. II that has been partially filled with gap filling CA glue. Part IV of this series will cover seam filling and repair techniques in further detail.
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