rotted door

Assuming it's repairable, what's wrong with exterior latex based wood filler? Works for me. You could also use auto body filler. It's advantages are that it will harden very quickly, even when it's 40F, you can put it in deep all at once instead of in layers and it's more durable. If you use that, I would sand as soon as it's hard, like in ~20 mins. It's still not at full hardness and easier.

The other question is why it rotted? Anything trapping water? Excessive rain getting to it for lack of a gutter? I had a steel door at a condo that rusted out. It was being hit by a lawn sprinkler that kept it wet many nights.