I used to have a 17' MFG outboard runabout when I lived near the bay, but it had a rotten transom core. We made a new one by laminating 3 sheets of marine plywood, and bowing them while they set up in the clamps. After cutting the old outer skin off with a router and cleaning out the mulch, I fastened the new core to the inner transom skin and contour sanded it to match the rest of the hull. I then laid up about 6 or 8 layers of cloth, with 3 or 4 extra layers around the motor mounts I poured Polyester Resin into the gap between the wood and the inner transom skin, and capped the wood across the top with a couple of layers of cloth, then sanded, filled, sanded again, and painted it all with gel-coat. I ran the boat another 5 years without any hull problems.
I believe the halo you are talking about is the thin layer of paraffin that seals the gel coat as it cures. It should be simple enough to remove with mineral spirits or the like, I had to do quite a bit of sanding to mine to make it look presentable, so I didn't even notice it.