I started glassing hungover - 1st mistake - and forgot to put patches on my fcs2 fin boxes - 1 layer of 4oz on there. Can i just put a patch on them now before the hotcoat?
Yes.
If you are nervous you can go slightly bigger with the patches.
Personally im not a big fan of fcs2 as i see boards coming through the workshop with cracks around the boxes just from the amount of pressure that needs to be used to install and remove fins
I typically add a single large patch over the entire tail area, covering all the fin boxes after lamination, after hot coat. I typically use a 10.5" center box and like to cap them. This accomplishes both the cap and patches over the side boxes.
I put such patches on top of the lam rather than under it, If under it, without a super thick hot coat, one can sand through the edges of the main lam.
I’d rather feather the edges of the patch than sand through the main lamination.