for those who have replaced your mechanical trim cables, what did you use to seal the transom hole?

Are you asking about what silicone to use or how to finish the outside of the boat off?
If silicone then i would use a fast cure 4200....make sure it says fast cure......the regular stuff takes 7 days to cure!
if your talking about finishing off the outside with some bling then something like a 45* pass through for the cable is usually whats done....or a stainless clam shell cover

We use these - https://www.diamondperformanceparts....roducts/wl-114

fountain put a plastic clam shell on the outside, was wondering if i should use 5200 fast cure or 4200 fast cure to seal the hole and inside the clam shell

Ditch fountains clamshell and use what diamond posted. That’s what I did with mine. Seal the fitting to the hull then I push some sealant inside the fitting before tightening. 4200 FC.