Weldbond and Titebond II will not loosen when wetted after they have dried. I use Weldbond a lot because for an aliphatic it sands reasonably well.

I've only used light spackle under paint, and so I don't have covering iron issues. Nice to hear about the Elmers--maybe I'll try it when my light spackle is gone. But the spackle has lasted me four years and still going, cost 5 bucks, and works fine--it'll be tough to beat. When I need strength I use Sig Epoxolite, but I've had some trouble with it getting gritty in the pot after it sits a couple of years.

Jim