When doing a epoxy repair will new epoxy chemically bond to old epoxy, or is it more reliant on a mechanical key, or a combination of both?
I did a fin repair a couple of days ago using 5 minute Araldite epoxy.
The result is good enough but the material has high porosity and wont take a high shine as it is slightly less than completely rock hard, even when cured properly and left for 24 hours. Will actual fiberglass epoxy repair material have less porosity and cure harder?

Epoxy will only bond chemically when poured or when the first later is green. Once its full hard there is no more chemical bonding.
It becomes a mechanical bond.
But who cares? as long as the surface is prepared properly it will stick better the **** to a blanket.

The 5minute glue is OK to plug a hole and get back out in the water, its very soft compared to proper lamenating epoxy, don't use it on fins in any case, so doesn't matter if it's a chem bond, it's just wrong, probably better to use some polyester resin on a fin, as there is no foam to eat into and get a better result

Regardless of how and why - Araldite is a glue and it not designed for prolonged immersion. It is water proof but is not designed to be wet all the time.
Next, 5min stuff will never be as good.

Polyester is not as strong, especially as a blob of just resin it will be brittle.

Best thing for fin repairs is a metal filled epoxy - the 24hr version - like Devcon 10110 or JB Weld. End of story.

If that is not avail, a marine laminating resin like West, BoteCote etc etc with a good filler like the "glue additive", aluminium dust, or (surprisingly good) a wood powder from belt sander. I mean wood dust that is like talcum powder consistency (not sawdust / filings / chips) and from real wood not MDF with all the glue n stuff in it. Sets hard as blazes.

Rough it up, get rid of the 5min half-set pog, soak in acetone to clean well then use one of the above.