Most older cars used flat glass for side and vent windows. Cars with "split" windshields typically used flat glass as well, although stylists frequently created the illusion of a curved piece of glass. Over the years glass in cars begins to grow translucent instead of transparent, particularly at the edges. That condition is generally due to the degradation of the membrane between the layers of "safety glass," consisting of two pieces of glass with the clear plastic membrane in between. Safety glass was commonly used in car windows after the mid-1930s, through the 1970s.
Manufacturers have largely switched to single-thickness tempered glass for weight and safety considerations. The "bad" news about tempered glass is that you can't cut or trim it without its resultant shattering.
The "good" news is that you can cut safety glass, and here's how. Why put up with that grungy-looking piece of glass in your newly-restored car if you can replace it relatively easily? Why not consider replacing clear glass with tinted glass? Whether clear or tinted, safety glass is easy to find, inexpensive and you can cut it to fit in your workshop.
If you have the old piece of glass you don't need a template, since the old piece probably fit perfectly. If a window is missing, however, you need to create a template. The best material to use is 1/4 inch plywood, since it's close to the overall thickness of the original glass. Besides, it's cheap and easy to cut.
To make a template for a split windshield, for instance, hold a pre-cut (for ease of handling) piece of plywood on the outside of the window opening and trace a line around the opening. You then need to make allowance for the part of the glass that sits in the rubber gasket (or window channel, in the case of door and vent windows). Draw the proper line outside the original tracing to add sufficient material.
Template for our XK120 windshield.
Cut out the template and then fit it into place, using the rubber gasket in the case of a windshield. Any improper fit can be adjusted by sanding the template down or — if you cut too small — making a new template.
Do you have a glass cutter? If not, go to the hardware store and invest $2.00 in one of the old-fashioned cutters (still the best!). Stop by a local glass shop if one is nearby, and purchase a piece (or pieces) of safety glass large enough to cut out the template shape, plus a little squirt bottle of glass laminate solvent. If no glass shop is handy, order some glass over the internet. If no solvent is available, buy a can of lighter fluid.
Using the template to transfer the shape to the glass.
As for cutting, you need to score both sides of the safety glass. Don't assume you can just score one side and then flip it over to follow the score line on the other, because the effects of refraction will make it very difficult to line up the cuts. Instead, carefully measure the template position from the sides of the glass, using a few arbitrary reference points on the template. Trace the template edges onto the glass (both sides) using a felt marker.
Glass with guideline from template on both sides.
Now we can cut. Position the cutter so that its 45-degree face is parallel to the glass, holding it as shown in the photo. Press down firmly and follow your line, moving forward (away from you). Listen carefully, because proper scoring of the glass surface is accompanied by a gently "hissing" sound, similar to a zipper being moved. Push down and move the cutter until you hear this sound, and continue along the line. Never cut over the line you've already scored!
The correct angle to hold the glass cutter. The base of the cutter (red line) should be parallel throughout the cut.
Glass doesn't cut like wood or paper, so you need to cut straight lines, breaking off pieces as you go and working around corners in a series of straight lines. Remember, you need to score both sides. Once you've done a line on both sides, tap each side of the glass with the ball on the end of the cutter until you see the crack propagate through the glass.
Tap the glass on the score with the ball end of the cutter. A crack should form along the score.
Douse the crack with lighter fluid, or alcohol.
Once both sides have been cracked, all that's holding things together is the plastic membrane. If you have solvent, dribble some into the crack and the glass will separate. If not, dribble some lighter fluid into the crack and light it. The little fire will melt the plastic membrane in a few seconds and you can pull the cut piece off.
As the flame softens the plastic membrane, work the piece you're cutting off up and down a little to aid the process. In a short while the piece will come off.
Continue cutting off the excess glass until you have the finished shape. Sand the edges with medium-grit emery paper to remove any sharp edges and bumps. There you have it, a new piece of glass!