We have wrought iron railings which were apparently mounted using the old method of placing them directly into holes in the concrete and sealing in place with something poured into the hole to hold them in place.

Problem: on several of the railings, one side of the square has rusted through (they are hollow, not solid) at the base of the post. The railings seem to be solid, aside from the rusted areas. My inclination is to fill and seal these areas with something like an auto body putty before I proceed to sand, prime and paint.

Does this sound doable? Is there a different product I should use?

Any and all suggestions appreciated. Thanks.