"Milky" or scratched airplane windows... any way to clean / polish them?

Hi Ted -

I use a lot of water and mild soap and also "Pronto Classic" by Johnson to clean my aircraft windows. Among German glider pilots there is a long tradition to use that stuff for bubble canopy polishing - gives 2 extra knots :-) Reducing glitter - I don't know - try it, cause it doesn't harm the plexiglas and is not to pricey. Amazon.de sells it for 4,34€ (2x250ml)

Though you can polish or coat the plexiglass, and make the surface more clean and smooth, the glitters are in the center of the plastic, and therefore largely unaffected by surface treatments. You might see a very slight improvement, if the very tiny cracks which comprise the glitter, are filled in with a liquid, but improvement is unlikely, as the cracks are so tiny, and any effect will be temporary anyway. When you see the glitters, it's time to replace the affected window. My experience is 30 years old, or 20 in the sun and the windows are done. even less, if they are bent into position - I'm replacing four right now...

Though never do it on aircraft plastic or windows, I have had great success with faded over car headlights by applying pure acetone to a paper paper towel (not a mystery material one), and giving ONE only horizontal wipe across the headlight. the layer of oxidized hazy plastic comes off, leaving a reasonably clear finish. Don't do more than one wipe though, of you'll definitely cause a negative appearance. This is not the problem with the aircraft windows, and certainly not the solution though.

I just got back from treating my aircraft windows with "Pronto Classic" (known as "Lemon Pledge" in the US with a different scent). It's the perfect stuff to get your windows to shine and it also works well on the rest of the aircraft.

Make sure to use a cotton rug though. Paper towels will scratch acrylic glass or polycarbonate.

Thanks for the responses. I knew it sounded too good to be true. I suppose the theory is that the wax penetrates the deep cracks of the crazing and reduces the reflectivity of the crack faces. I've replaced the screens twice in my 30 yr ownership of this Robin and it is a major undertaking. The present crazing is very localised and is invisible unless the sun is at a certain angle.

I'll probably give the wax a try. It can't do any harm.

Could it be that the crazing is caused by some chemical?

I saw a huge amount of that in the planes I trained in, but so far mine has none and it's 12 years old.

OTOH it is hangared. If UV was causing it then a cover would be the answer.

The "glitter" to which Ted refers is not the same as "crazing". Crazing can be presumed to be surface damage to the plastic. Glitter is actually a whole lot if tiny stress cracks in a stressed are of the window. Some PlexiGlass and Lexan windows are forced into a curve at installation, rather than being preformed to that shape. Even some preformed windows get a little force for the fit. Over time, and UV does not help, they just succumb to the stress. Eventually, much larger cracks will appear.

However, to take this a layer deeper, from J.E. Gordon's excellent book "Structures" we learn that it requires energy to create a surface. So, no energy, no new surface = no crack increase.

So, the plastic window finds its happy place, and possibly gets some glitter cracks as it internally redistributes stress as it does so, it may settle, and stay that way for a long time - as long as you don't flex it or push on it! The glitter is still annoying though.

Moulded windshields (Cessna, for example) inevitably have optical imperfections associated with forming. In some, if you look carefully, you can see the optical pattern of the "V" notch and filler piece in the felt of the male mould, in the wing root area. The other windshield philosophy is to just bend the plastic to fit. Optically better, and simpler (no tooling and extra process), but it's going to stress crack over time - just the cost of maintaining the plane.

I'm in the middle of replacing the four Lexan panes which form the side and top windows of my Teal. Over an unknown number of years, the 1/8" upper ones had got a lot of glitter, and crazing. It is time. When I took the panes out of their curved frames after all those years, they sprung back to flat right away. I'll be happy for the new windows - though to get the four two 2 foot square windows I need, I have to buy rather expensive 4 by 8 foot sheets of Lexan. But now, I'm good for as long as I own the plane!

Just to refresh, PlexiGlass/Perspex is acrylic, less costly, and more common in GA aircraft. It will splinter as it breaks. It can be sanded down, and polished back to a mirror finish with the proper methods. Lexan is polycarbonate. It is more expensive, will not break, and cannot be polished once its finish is damaged.

If you are drilling PlexiGlass, there is a special way that the drill is to be sharpened, to prevent pulling through and cracking the plastic. If you are sawing PlexiGlass, you must not use a reciprocating saw of any type. Either circular saw, or bandsaw.

Generally, for GA airplanes, I have found it is cheaper to simply replace windows, than the time and effort to revive them.

My knowledge of industrial plastics comes only from designing various mouldings at work (ABS, acetal resin, polycarbonate, and some special "bendy" stuff whose name escapes me which we buy 1000kg a time in a custom colour from Italy and then ship all the way back to Italy to be moulded ) but surely the cost of the material is utterly immaterial in this application, so why doesn't everybody use polycarbonate? It's really tough, naturally meets V-0 flame retardancy, and should be better with UV than "plexiglass" which (Perspex etc) is basically cheap rubbish used for shop signs etc.

Polycarbonate/Lexan scratches much more easily than Acrylic/Plexiglass. There are articles on the web comparing the two, but basically polycarbonate is tougher and much more fracture resistant when being drilled and cut, but scratches much more easily once installed.

There are WW II aircraft that still fly with their original clear acrylic transparencies because they haven't discolored and can be refinished. As Pilot DAR says, Polycarbonate cannot be sanded/refinished because its too soft - the same quality that gives it good fracture toughness.

Another thing to know about polycarbonate is that if you get the tiniest drop of thread locking compound (Lock-tite) on it, the next day it will be cracked and crazed in that area. Useful to remember when installing vent windows - I learned the hard way.

The normal way to remove scratches is to polish them out, but that removes material and results in a visible amount of optical distortion around the area where material was removed.

With car (glass) windows, there is a process where a dent (typically a stone chip) is filled-in and the result does not change the thickness of the glass.

Is there such a process for our plastic windows?