How to make water resistant seal between 2 pieces of plastic

[Wilson__]

Seem cell phone back factory cover use some 'glue, soft and sticky' material that are press-cut for shape of the seal area??

Seem a short-cut, when our phone battery/cell was replaced, the repair person used "some kind of" glue applicator (tooth paste casing, just smaller and with a pointed nose) to spread the 'glue' around the edge and pressed the back plastic plate back to phone body.  Could it be those white silicone that are used as sealant for windows glass, bath room and fish tank? 

Many thanks

[BILLPOD]

Good Morning Wilson,    What you're describing sounds like RTV silicone.
RTV silicone (room-temperature-vulcanizing silicone) is a type of silicone rubber that cures at room temperature.

[Wilson__]

Many thanks.  Is RTV used as bath room sealant?  I did not recall the name.  When i brought from hardware store, it is 30cm long cylinder with matching hand pusher.  Give acidic smell and cure in 24 hours. It was white color paste. 

[jpanhalt]

If the original repair did not smell like vinegar, then the silicone that does is not the same.  Search on "neutral cure RTV silicone."  There's a huge variety, and they come in clear as well as colors.

[DavidAlfa]

Search in AliExpress for E6000/8000/9000 T6000/8000/9000 glue.
Use black glue for black phone, clear for white phone.

I've fixed many, many phones with it
20ml is more than enough for plenty of times - I used 110ml ones, would last easily 100 phones if not 200.

Put some around the edge of the phone and where required, leave drying for 5 minutes, then put the cover.
Very little is required, like a thin noodle, be careful around cameras, microphones, speakers, buttons, card slots...

If you attach it too soon, the glue will be too runny and spread everywhere.
After those 5 minutes it'll have thickened and become a sticky rubber, put the cover and give a smooth massage, then place the phone flat in the table and put something flat and heavy on top(~3kg).
After 5 minutes remove the weight and check for oozing around the edges, clean with alcohol and put the weight again, leave for  at least two hours, you can leave more if you want, I would leave overnight when possible.

Even when dry, alcohol will remove it, but will also tear it off and  break the seal (Will do like a endless thread, the more you clean the more it comes out), so that's why it's better to clean it off when still wet.

The cover will be easily opened with some heating, so not hard and permanent like silicone or other glues.

Needless to say you must clean all the old glue first!

[jpanhalt]

I use E6000 (USA stores) a lot.   Source: https://eclecticproducts.com/ 

It used to be called shoe goo; now those are shown as different products.  It has a characteristic, sweet organic odor from perchloroethylene (PERC) used as solvent (https://eclecticproducts.com/wp-content/uploads/2024/05/E6-Craft-TDS-USA-CAN-MEX-ME-AU-NZ_REV-9.pdf).

Unlike RTV silicone, it has a very high peel strength.  That's why it works well on rubber soled shoes and in my use to attach pushrod sleeves to fiberglass fuselages in model sailplanes.  It's also hard to remove, but I have never tried PERC.  Silicone tends to come off in long strings and is not soluble in anything.

Both should work, but I would rather re-open a case sealed with silicone than with E6000.