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1. The ones that are any good end to have very strong acids (e.g. OneShot) or very strong alkalis like Sodium Hydroxide.
2. Don't think I'd want to give them to anybody who:
2a. Might sue in the event of an accident.
2b. Might not use them properly and cause damage to fixtures or fittings.
The stuff you get in the shops is pretty dilute, and hence fairly safe. Buy them a pair of cheap goggles while you're at it. Anything with sodium hydroxide in the ingredients is good. Sodium hypochlorite is a bonus. I'm a rare oddity in being both a plumber and a chemistry graduate!
Plungers cause all kind of problems, as the compression they generate can wreak havoc on pushfit and compression connections, such as the ones that connect the trap to the pipework. There's always a better solution.
If you want to give them a decent means of mechanically unblocking waste pipework, buy them a TITAN wetvac from Screwfix for around £50. The vacuum in the pipework is unlikely to cause leaks. They'll need to cover the overflow of any sink or basin in order for it to be effective if they're sucking out from the waste AKA plughole
I asked a drain engineer what he does when he gets a shower tray block or sink thats hard to get to waste and he said boiling water with a strong bleech and leave in trap , it does seem to do the trick .
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