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Even though it's super expensive it doesn't go off in the tube once you've opened it. At least I've had some open for a couple of years and it was fine. In the situation you have there you could opt for the cheaper more liquid Timbacare or County Chemicals who are the company behind Sealants Online have launched their own competing product which I've yet to try.
Repaircare itself is a brilliant range of products but it does feel expensive.
Ta will have a look.