Assuming the goal is to make the car durable and safe, you MUST weld in metal in the rocker area as it is definitely structural. I've done a lot of body work over the past 50+ years and fiberglass and paint WILL NOT fix this kind of damage, just make it look OK. I can't tell you how many of those patches I've cut out and replaced with metal patches that I've bought or fabricated after they've cracked away from the body structure. The body will flex even more with an improper repair and it WILL pop out a bonded repair, including one held in with rivets.

Now, there are structural bonding agents that can work but they must be used with VERY CLEAN metal and they're painfully expensive. I have the Lord Fusor system with was (at the time I bought my applicator) about the cheapest one out there and it was still pricey. This is the repair system used for body panels that are fused from the factory, as many are now starting to be - but this is not an easy short-cut for proper repairs.

You can use the fiberglass & paint process in non-structural areas with success. For example, I once did a Trans Am that had bullet holes in the sail panel as the result of police gunfire (it was used in a robbery). That section of the top has limited structural stress and I didn't want to replace the headliner so I did the glass and it held up just fine.

POR-15 and those types of products work fine if used as directed and if you're after a finish (not structure) on a part that was rusted. That application is really what they're made for.

Dan