I have two plexi glass acrylic T tops on my 92 Firebird that have developed
fine hairline cracks deep in the smoke colored plexi glass. I've sent away for a plexi glass restoration kit which included several grades of sanding paper
and a plexi restoration chemical. After many hours of fine sanding I still am
unable to sand down to the real deep cracks. Does any one have a solution
to this one?

 

I guess I should state that I make windshields and windows for aircraft, which are made from Acrylic, AKA Plexiglas(r).

The cracks are called Crazing, what is causing it is the plastic is deteriorating. Crazing is caused from several things, the most abundant is from the UV rays in sunlight. Secondly items like Windex, and typical household glass cleaners shorten the life of the plastic. Unfortunately its a property of the material and there is really nothing new under the sun that is overall better than Acrylics.

Crazing usually starts as haziness, and then develops the fine cracks that you are experienceing now, when they start to develop they are acutally just under the surface, and as they get progressively worse they work their way to the surface and eventually it is a place where a real crack can start. Since acrylic is notch sensative it essentually is a means to the end.

What can you do?
1) you can continue your work and eventually sand out the crazing as your doing now, it takes a long time, if you have ever done some paint work to sand through a couple mills of paint is kind of difficult, although not impossible. Likewise the crazing you are seeing is imbedded into the material, and you might have to remove .030" of material depending on the severity of the crazing.

2) Replace them with Glass Tops, - readily available at junkyards, I can usually get them for around $20 to $70 each

3) replace the Plastic panel, Unfortunately Leximar no longer makes them to my knowledge, an no one else makes them either.

I have considered making replacement panels, out of the same grade material that we use on aircraft, but by the time we get done you will be looking at around $200.00 to $300.00 each side, by the time I Make the molds, buy the material, form it, and mill the edge thats about what it would cost. Then I would have to trust that the owner of the car is able to actually install the window properly and not complain because they are inept. If I chose to make them from cheeper material then I would have people complaining about optics... Then the complaints about the price, some car people are like some boat people they tend to be cheep and are not willing to pay for the work you have done...

So, there is your options, I hope I did not discourage you. The panels are able to be worked out, but its going to take some effort.

John Zofko
Marketing Manager
Great Lakes Aero Products, Inc.
www.glapinc.com

 

I'll try sanding for those hard to get areas:Thanks,Walt

 

I would just go with glass tops. Make sure you get the right pin type. I had a set of plexiglass tops that were pretty ugly, and I even had one melt on a hot august day (my fault, I left the car closed up while I was out of town and we had 2 or 3 110+* days in a row). Now I have a set of glass and they look so much nicer, both inside, and out. Not to mention you can actually see the sky in the daytime, which is cool. At night its like having a mirror on the ceiling.

 

novus 3 step plastic conditioner....made for aircraft windows

 

If you are trying to sand out a small area of crazing do NOT just focus on one small area. If you do you will create a distortion or a lens effect which may not be desired... If you don't care most people will not notice because typically you do not look through the panels, but I want you to be concious of it.

John