Forget Rain-X ... that crap never lasts. Get some glass sealant. Stuff works wonders. Adam's is about $20 for 4oz but it lasts forever. Application only takes a few drops of the stuff. Ideally you would reapply every 1-3 months depending on the amount of driving you did in that time and any rain encountered.
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2011 370Z 6MT Sport Gun Metallic | ARC | CJM | Ecutek | FI | Fujimura | R2C | SPL | Stillen | TWM | Z1 | ZSpeed | |