What is a flap?” A flap is a sliver of cured urethane lying on top of another piece of cured urethane, which can cause a leak or bonding problems. Any time you are replacing a windshield and stripping back the existing bead, flaps in the urethane must be eliminated.

A flap can be caused by the cut-out procedure, strip-out procedure or by a combination of the two.

A flap can be caused by the cut-out procedure, strip-out procedure or by a combination of the two. Often, a flap is not noticed by the technician due to insufficient light, compressed timelines or the (proper) practice of not touching the bead after the strip-out procedure.

We never want to create a flap, but it can be easy to do. For example, using a cut-out tool in one direction and then finishing the cut-out action in the opposite direction can cause a flap where the start and stop points overlap. Most of the time, this flap is eliminated by the strip-out action of the full strip process, but if the flap is deep enough it can go unnoticed. The sliver left behind is almost invisible to the technician and easily missed.

If the flap is deep enough it can go unnoticed.

A flap can also be caused at the start and end point of the strip-out procedure, where the technician trims back the existing urethane bead. Again, a flap can be created if the tech strips the urethane in one direction and then finishes in the opposite direction.

If a flap is not discovered and eliminated, the new fresh bead of urethane is applied right over it, which can cause an entry way for water into the vehicle. The flap also creates a bond problem.

The first step in eliminating flaps is to realize they exist, and to look for them. Use your strip-out tool to find possible flaps within your trimmed-back bead. Gently run the tool over suspected areas, and try to catch the blade on the loose flaps. Once found, cut them off.

Try to catch the blade on the loose flaps.

I know it’s easier to use your gloved hand to find the flaps, but you run the risk of contaminating the freshly exposed existing bead with your gloves. If you must use your hand to find and eliminate a flap, make sure you wipe the surface clean according to your adhesive manufacturer’s instructions.

The flap leak can be difficult to fix. With other leaks, a little bead of fresh urethane placed over the leak area will work. However, with a “flap leak” this technique may only shift the water leak to another location. The best way to fix the leak is to remove and install the windshield. However, if you decide not to do a full removal and installation, I suggest the repair bead be extended the entire length of the roofline and down the sides an inch or two. This will divert the water and have a better chance of sealing it for good.

You run the risk of contaminating the freshly exposed existing bead with your gloves.

Don’t forget, the best thing to do is not have flaps in the first place. Take the time to look and eliminate them.