i got this http://stanstiresealant.com/ i have used it in my mountain bike for years and never had a puncher since, i have also had it in my car when a 6" nail went in and after 8 months it just turns to latex rubber, also use it i my off road bikes.
and only £10 a bottle that does about 4 bikes

i can also get hold of it if anyone needs some.
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