Working with new adhesive can be a bit tricky but there are good do-it-yourself videos that can give you some good tips on how to apply the new gasket before re-sealing the phone
That is actually covered in the video. Did you actually watch it? If water resistant is important to you then what you would do is purchase new gasket material so that after you have opened up the phone and removed all the existing gasket you would apply the new gasket and then finish up and re-seal your phone. As to how water resistant it is after re-sealing the phone all depends on how careful you are and how exact your work is.
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