I'm curious as to why you use tack when assembling the helmet?
I am also curious about the tack. What's your process?
The tack is used for masking. You can buy
masking putty at some hobby stores and on-line. Its a trick I picked up from guys that do army tank model kits. You can push tack-putty into areas you just couldn't tape. I find this cheap dollar store putty to work just as well for this use-case.
Like the louvers and details on the front of the helmet, or on soldier figures where you want to mask off at the belt or whatever where it would be impossible to tape.
In this case where I'm using it... the two halves of the helmet don't make a perfect flush join. So I mask along the seam, then glue the two together. The mask & putty act like a dam or barrier so the epoxy and resin don't go beyond the seam to be filled.
Then flip the helmet over and work from the inside. Fill the gap with your favorite adhesive for this sort of work.
Now its just filler putty, sand, primer as usual until you can't tell there is a seam.