see the line of what like some coating or something is breaking ? i dont know what it is but it makes the headlights look dirty or fogged , and where this stuff has peeled off the headlight looks brand new
the headlights are the original factory stanley ones ,its a 1999 ej6 civic coupe

 

From past experience , it is normal . Too much sun will cause it . It will get worse with more sun exposure. Always thought it was for UV protection

 

Tape off the hood and bumper so you don't ruin them. Wet sand the headlight with super-fine grit sandpaper. Dry the headlight off and apply at least several coats of clear spray paint. Remove tape and you're done.

 

what number or grain level of sand paper should i use ?

 

The finer the better. Although, you can always get away with a little coarser grit if that's all you have on hand because the clear spray paint fills in the scratches.

This will last for several years- the more coats of clear the longer in generally lasts.