Making Plastic Windows Clear Again

H2OBO has a pilot house and large plastic deadlights through which we look to steer from our inside helm station.  During the refit, I installed 1/2" thick polycarbonate plastic (Lexan is an example of a brand), rather than acrylic (Plexiglass), into the openings for its shatter resistance.  Unfortunately, polycarbonate is less UV resistant and softer than acrylic, so with time it will become less transparent, more scratched and need to be polished.  With hindsight, I should have installed 1/2" acrylic, which would have been sufficiently strong enough to do the job and require less maintenance.

It is a tedious job, but I have done it a few times now with excellent results.  What you will need to do is to wet-sand the plastic first and then polish it.  The opacity is caused by three things: scratches, oxidation, and crazing. Crazing looks like cracks in the plastic and goes a little deeper than oxidation.  Deep scratches might be difficult to remove, but they can be made less obvious in any case.  Depending on the severity of the problem you will want to start with the correct grit of sandpaper.  For light crazing, I started with 600 or 800 grit paper, which I bought at an auto-body repair supply store.  If there is just oxidation, you can try starting with 1000, 1200 or 1500 grit.  During each course of sanding, you want to keep things wet, not press too hard and sand until the plastic is uniformly sanded.  Don't move the sander around too quickly; otherwise, you will have undesirable swirling patterns.  Check that there are no foreign objects under the pad that might scratch the plastic.  Wipe off the residue with a soft cloth and water often to check on your progress.

I use a quarter-sheet electric sander and then a random orbital grinder with a soft, sponge pad set at a medium speed.  It is important NOT to heat the plastic as this will cause further optical distortion, so during the sanding process I use a spray bottle with water in it to keep the plastic wet while sanding, and during the polishing process the lower speed on the grinder.

Once you are ready to move on to the next grit, clean the surface with the cloth and fresh water.  The sandpaper regime I use is the following: 800, 1000, 1200, 1500, 2000, and 2500 grit.  I have not been able to find any finer sandpaper than 2500.  After the sanding is complete, clean the plastic and get ready to polish.

You will need to get a plastic polishing compound like Meguires or Novus.  I have used both with similar results.  Usually they come in kits with two levels of grit: a scratch remover and a polisher - use both for optimal results.  My Porter Cable Random Orbital grinder outfitted with a soft sponge serves well, but I have also used a lambskin polishing pad.successfully.  Carefully follow the instructions for the polishing compound and you will get good results.  Afterwards, clean off the residue with a soft, clean cloth, and you will have beautifully transparent windows.

The clarity of the plastic can now be maintained with just occasional polishing.  If you neglect to do anything for too long, like during winter lay-up, sanding will be required again, albeit starting with a finer grit than the first time. I am going to try applying a polyester film that is used to protect the interior of homes from UV damage.  This product is meant to be applied on the inside of glass windows, but I am going to try it on the outside to protect the plastic from UV and scratches.  I will let you know how that turns out.