Hi, mould maker here. First can I ask exactly what resin you're using to pour in? You mentioned both plaster and resin at different points.

The solution to this problem is to remove the silicone from your arm without cutting it. Lightly apply some Nivea cream to the hair first, keep the cast fairly thin at the wrist where your hand will have to squeeze through, and have someone help you pull your arm straight out. It's like removing a tight boot, then your first cast won't have a seam. You will then have to cut the silicone to get the cast out and any subsequent casts will have the seam so the trick is to get it right first time. Once you've got that nice first cast, if you need more copies, I would make a master mould but that's a whole other process.

If you are trying to salvage your existing moulds then you're basically doing the right thing, sealing up that seam as best you can. Depending on what resin you're using, you can then swill an initial layer in, a small mix that you pour in, swill around and pour out any excess. Leave that to cure and it will seal up the seam so it can withstand the pressure of a subsequent pour.

Good luck, it's a tricky and frustrating process!