i've tried it personally..wad i can say is,RM30 is way too cheap if u see the results..the windscreen might seems very clean to most of our eyes but after u apply the watermark powder,everything comes out..
it doesnt juz remove watermark,it remove wiper marks as well..i wish i could buy more n stock up for future use.haha..
regarding on how many packets needed for whole car,cant give any comment. xD since ra2boy already mentioned 4packs(maximum result)
i help my fren apply on his windscreen,he's been using RainX all the time,got beading n repelling effect as u all noe,after apply the watermark,it strip down everything to its 'virgin' state...i'll post pictures n 1 video to show the results.
anyway, definitely a very good product!

This is the wiper mark that are invisible to naked eye..everything comes out after initial apply of Rainaway Watermark Remover
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a very clear comparison of Before/After..
use a sponge to rub on the windscreen until u get results like the one on the left then ur done..ready to rinse off.
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Crystal Clear..follow by Rainaway Nanocoat then ur windscreen is protected throughout the year!
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cant seems to post the video..