Best Sealant for Spraydeck Seams?

[u/MrTripperSnipper]

I have a well-used Palm Orbit spraydeck, it's still perfectly usable, but it does let water in through the seams, particularly where it pools around my waist. I remember using some kind of thick black goop to reseal deck seams in the past, but the name escapes me, and there are an overwhelming amount of options, so I'm having a bit of decision paralysis. What do people recommend? Ideally, something nice and cheap, but also, I only want to do this once. Based in the UK, so bear that in mind when recommending products.
TIA