First, you need to warm the car window glass. You can accomplish this using a hair dryer or you can park the car in open sun for a few hours at least. It would better to use a hair dryer if you need to get the job done rather quickly. When the car window gets warmer, air trapped between the window glass and the tint expands, thus making even the smallest of bubbles clearly visible.
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Using the water spray bottle, dampen the surface of the window tint with a small amount of water. Remember that you just need to dampen the window tint, not wash it, so do not use too much water because it will again condense the expanded air and you will not be able to remove the air bubbles properly.
Now puncture a hole in each air bubble. Use a small sewing needle for the purpose. Make sure that you do not move the needle while puncturing the bubbled tint because if you do so, you may rip the tint in which you will have to replace it with a new one.
Now it is time to remove the air from the bubbles. Carefully sweep across the tint surface with the plastic card held at an angle of 45 degrees. Do not apply too much pressure. Just the right amount of pressure to remove the air would be good enough. If you apply too much pressure, you risk the chances of tearing the tint.
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Using a lint-free cloth, gently dry the surface of the window. Give the tint a final look and make sure all the bubbles are gone and none remain. In case there are any bubbles left, you must have missed puncturing them and you need to repeat the process.