Ah yes, you bring back many memories. None of them very good.
My best tool for getting that plywood off was a grinder with a sanding disc attachment. I used this and got some really heavy duty sand paper. I’d recommend templating before you get too far into demo. That could have saved me lots and lots of time and materials.
https://www.homedepot.com/p/DIABLO-4...S01G/202830887
That red stuff is potter putty. Essentially peanut butter fiberglass. Teak looks really good. Mine was in far worse shape. Yours should look even better. That Pettit flagship varnish is thick enough to cover the grain and level it out with numerous coats applied. You must seal the wood first or it will crack on you. Shellac makes a good sealer. You can buy many brands of sealers too. But the dirty secret is most of them are alcohol based (just like shellac).