When do I add Isenglass?

[oldjiver]

If you rack onto it into a secondary and leave for 24hrs then i assume you cant batch prime or you'd have to rack into a 3rd FV?[/quote
No I dont rack into a secondary most of the time. I ferment to finish in an airtight bin and reseal after adding finings for 1 day then rack to batch prime. 90% of the yeast has fallen out leaving just enough to give a secondary fermentation.

So how do you mix it in fully without disturbing the trub?

I take out about a litre in a jug, then pour a little at a time onto the isinglass gell in a cup and stir. Then tip the cup back into the litre jug and stir that gently back into the FV. The trub/yeast in the bottom is hardly disturbed and by the next day has settled along with 90% of the yeast