Advice on sealant

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I’ve up-cycled the slate bedding from a snooker table, by using it as a a work top for a BBQ I built in the garden.
I was really pleased with my idea when I first installed it.
But it’s been a few years now and it’s looking a bit shabby, it’s gone grey and had moss forming on it.
So I have sanded it down and got rid of all the moss, but I want it to be glossy like when it was first installed.
Is there a product that would restore it to its shinny former state.
It looks great in the rain, but when it dries it returns to a dull grey finish.

As you are looking for a glossy finish, I would recommend using the High gloss PU (link below) this will give you a good glossy finish (like when it is wet) and it is a very hard wearing resin so it will last many, many years before it would need re coating.

Drew Palin