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Polishing perspex and plexiglass

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Plexiglass/perspex is a very soft type of plastic (Polymethyl methacrylate) that is very rarely used. Just like glass or paintwork, it can suffer from surface imperfections. Swirls and scratches can be very visible because it is fully transparent.

Why use perspex

Perspex offers several advantages compared to glass. Some of these characteristics make it more suitable for certain applications, others simply make it more cost-effective or easy to use without really needing the added benefits of the material.

  • It is lighter then glass
  • It doesn’t shatter when it breaks
  • It can easily be molded into many different shapes
  • It is more translucent then glass
  • It has a higher impact strength then glass (but lower then polycarbonate)
  • It is cheaper to create in volume then glass
  • It transmits more light then normal automotive glass
  • It can withstand more pressure then glass
  • The surface is easy to work with, for example when it needs to painted
  • Custom build roof windows

Surface imperfections in perspex

Although perspex is very easy to work with, and has several applications, it is much softer then glass. This results in a surface that is scratched more easily then glass. In the use for visors for helmets, it can be difficult to look through the visor. To many swirls will prevent the light from passing through the material. The hardness depends on the exact composition of the material, in layman terms it is often said that cheaper perspex is softer then more expensive perspex. When the material has become scratched/swirled to much, it can become either difficult to look through or it looses its shine. Like many other materials, perspex can also show oxidation, which is a common reason to polish headlights. If to much light from the headlights is prevented to pass thourgh the perspex, it can cause a failure for a MOT. If to much light is redirected in the wrong direction, it can also cause a failure for a MOT.

Polishing perspex

When perspex is showing swirls, RDS, marring and other surface imperfections, it might be time to polish the surface. Although the general process is not very different to that of polishing glass or paintwork, some care and attention is needed to get a perfect finish. Some things to keep in mind are:

  • Be careful with generating heat, because it can warp and bend the panel
  • Be carefull with to high rotational speeds, perspex heats up quickly due to friction
  • Use a bit more liquid while polishing to keep the friction low
  • Due to the very soft surface, you might want to focus on finishing with a very soft polish pad and finishing polish
  • A Dual Action is recommended instead of a rotary
  • Use a liquid (like panel wipe) when wiping off the polish residue, you are likely to cause marring otherwise
  • You can still clay the panel before polishing
  • Although wetsanding is possible, it is not recommended because it is easy to remove to much and to generate a lot of heat while doing it
  • Special plastic polish will aid in achieving a better finish
  • DAT polishes are more likely to aid in achieving a smooth finish
  • Clean the pads regularly, there can be a quick buildup of surface particles in the pad, which could cause marring while polishing
  • To much pressure can cause surface imperfections
  • If you notice that a normal sealant doesn’t perform as it should, try a glass sealant. These sometimes work better

A common polishing process for perspex might look like this:

  • Start with washing and cleaning the surface like normal
  • Clay the surface but use a liberal amount of claylube
  • Wipe-dry the surface
  • Start with the least aggressive combination of polishing product and polishpad
  • Set the machine to its lowest speed and slowly spread the polish evenly over the surface
  • While making the criss-cross pattern, slowly turn the speed up till roughly 50% of the max setting
  • The key is to work slowly and carefully with a low speed, always try to avoid putting to much pressure on the surface!
  • Do not let the polish dry up completely, aid a spritz of water when it has become fairly dry
  • Take your time to work in the polish properly
  • When done polishing, spray a polish-residue remover (such as IPA) over the surface and carefully wipe it off with a clean and unused MF cloth
  • If in doubt while judging the result, gently wipe the surface with a glass cleaner
  • Only if sure that a second pass won’t make a difference, go up 1 step in aggressiveness
  • When done polishing, apply a good coating or sealant to protect it from the elements

Perspex in vehicles

Although perspex can be used in the application below, it does not mean that they are always made from perspex. For example: cheaper and older headlights can have a protector made from perspex, even though the newer ones are often made from polycarbonate due to its higher impact strength. Polycarbonate is a common substitute for perspex.

  • Headlights
  • Taillights
  • Transparent cover for speedometer
  • Screen for dashboard clock
  • Windows in rally/race vehicles
  • Visor of a helmet
  • Windscreen on motorcycles
  • Indicators
  • External sunvisors
  • Convertible windscreens
  • Custom build parts for show cars

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