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How to Remove Scratches and Defects from Clear Parts

Introduction

Getting a shiny scratch-free cockpit canopy is an essential element on a good aircraft model.  The cockpit is an area that the eyes are naturally drawn to, it is the ‘jewel in the crown’ and no matter how good the rest of the model might be, the overall impression will be greatly diminished is the cockpit canopy is dull or scratched.

Unfortunately, this is often the state that modellers find clear parts when they open the kit box for the first time.  Fortunately, it is possible to correct almost all problems with clear cockpit canopies.  This method applies equally to any clear parts such as windscreens on cars.

Method

Eurofighter canopy 1 This is how the canopy started out, with a very visible mold line across the top of the entire canopy.  It actually shows up more in real life than in this photograph.

We will use the canopy from Revell’s 1/72nd Eurofighter Typhoon as an example.  The canopy was molded with a prominent seam line right across the dome.  There were also several small vertical scratches on one side.

The first job is to remove the seam line and any scratches.  This was done with gently rubbing with 800 grit ‘Wet & Dry’ (W&D) paper.  This hard wearing sandpaper can be used dry like normal sandpaper or with water.  Although a little messy, it works best with water so all of the sanding was done wet.  As an alternative to sanding the mould line away it could have been done by carefully scraping with a craft knife.  Whatever method is used, it is important to preserve the ridge that marks the demarcation between the clear canopy and the canopy frame.

Eurofighter canopy 2 Once the sanding has been completed the canopy looks like this – much worse than before work started.  Don’t panic, this is all part of the plan.

Once the raised mould line has been removed, together with any other fine scratches, a process of bringing the canopy back to a good shine begins.  This is done by using progressively finer sanding materials.

Those areas that had been sanded were now sanded once more, but this time with 1000 grit W&D paper.  This was followed by another sanding with 1200 grit W&D. I would have preferred to use 2000 grit W&D, but was unable to obtain any. On this sanding, the entire canopy was sanded, not just the parts where scratches had been removed.  You do not normally need to spend more than a couple of minutes on each stage of the sanding, but make sure that the entire canopy gets an even treatment and use a gentle circular motion.

If the canopy is delicate, you may wish to support it by gently pressing Blu-Tak or damp paper towel underneath.

Brasso metal polish At the end of this process the canopy should be perfectly smooth.  Unfortunately, it will also be cloudy arising from the myriad of microscopic scratches created by the fine W&D paper.  The next stage is to polish the canopy.

Eurofighter canopy 3 The canopy after polishing.

The canopy polishing in this case was done in two stages.  The first involved using Brasso metal polish and a soft cloth.  After a few minutes the canopy looked a lot clearer although the polish appeared to etch the plastic in a few places leaving it slightly cloudy.  The final stage was to use toothpaste with a soft cloth and this very quickly brought the canopy up to a good shine and clarity.

Although I used metal polish followed by toothpaste, I have heard of other modellers using just one or the other to get good results.  The results you get will depend on the quality and hardness of clear plastic used by the kit manufacturer, the type and strength of the polish and toothpaste.  The point is that the canopy can be brought back to a clear shiny surface providing a very fine polish is used.  Just make sure that the polishing compound used does not contain solvents or chemicals that might damage or soften the clear plastic.

Canopy polishing diagram Make sure that the rotation of a motor tool is away from the edge of the canopy.

Using a soft polishing attachment on a small motor tool is a very quick and easy way to polish the canopy but extreme care is needed.  It should be used on a low speed very lightly to avoid generating a lot of heat and melting the clear plastic.  Also care needs to be taken to avoid touching any part of the canopy with the spinning drill bit – easier said than done – as the slightest touch will create a deep scratch almost impossible to remove.  Finally, make sure that the direction of rotation is always away from any edges or the polishing head may grip the edge of the canopy and rip it out of your hands

 

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One comment

  1. The brasso is unlikely to be “etching the plastic”, it is probably just failing to remove all the scratches created by the previous abrasive and you may need to use an intermediate grade of abrasive before the brasso which is much more efficient in the long run. The danger is that you spend so long scrubbing away with that soft cloth and brasso, you blur any details and round edges. Using motor polishing will be catastrophic for crisp detail and really should be avoided. You also should mention that between each grade of polishing material, the part should be scrupulously washed in warm soapy water in order to remove stray abrasive which will otherwise sabotage your next step.

    Have you tried 3M abrasive film? This is available in a number of grades (40 micron down to 3 micron), right down to a grade that is so fine (much finer than Brasso which is actually relatively coarse), it feels silky smooth and leaves a mirror shine. Cousinsuk.com sell a handy starter pack with one sheet of each. It is also very durable and washable for reuse.

    I’m not an experienced model maker, but I’ve done a hell of a lot of polishing during my training as a watchmaker – we often have to polish irreplaceable acrylic watch ‘glasses’.