Automotive Plastics/cracked headlight and moisture
BillT wrote at 2012-11-08 01:06:47
If the lens on the headlight is made of polycarbonate then the cracks can be repaired by solvent bonding. You just need a solvent that will start to dissolve the polycarbonate.
Methylene chloride will work. So will methyl ethyl keytone (MEK) or lacquer thinner. I have listed them in the order of preference for use, which is also the order of volatility. The methylene chloride will evaporate almost instantly.
The nice thing about solvent bonding is that if you have a plastic that cannot be disolved by your chosen solvent, the solvent just evaporates and you can try something else.
The easiest way to apply the solvent is by taking up a drop or two in a very small tube. When the tube is touched to the crack the solvent will wick into the crack by capillary action.
As the solvent is evaporating and migrating into the plastic check to see whether the crack is being sealed closed. You can add more solvent as required if the crack is not completely closed.
Be careful not to get solvent where you do not want it on the face of the lens. If it runs where you do not want it, do not try to wipe the excess off. This will only make the blemish worse. Just tip the part so the drip runs away from critical areas and/or blow on the solvent to evaporate it more quickly.
A drip of solvent that has not been touched will leave a blemish that looks like a drip of clear varnish, but if you wipe it the surface will be badly marred.