Straight out of the bottle. Windex is basically water, isopropyl alcohol and ammonia. Diluting it further will only weaken the solution.
I buy ZEP brand from Home Depot (and other stores, HD is just the most convenient). It's much cheaper, available by the gallon, works every bit as well, and I save the liquid for the next "less than stellar" paint job.
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Oooops, just noticed you're in Karachi, it might be quite a way to the closest Home Depot!
Windex is approximately 70% Water, 5% Ammonia, and the rest is Isopropyl Alcohol. You could try spraying on the solution and give it a bit of a scrub with a soft brush, rather than soak.
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