HELLO RUST! - Fiberglass Is In My Future

I started working on cleaning up the cowl section of the Fiat, when to my surprise I found a big old rust hole! The rest of the interior is so great, it's a shame this had to happen. My only option is to clean up the area really well, remove as much rust as I can, seal it, and lay out some fiberglass. It will be an afternoon project, but I'll need to wait for the weather to get warmer. I'd like to do this outside. I'd also like to pull out the heater flap and figure out a way to get it more solid. It's falling apart.