OK to use Iron Out in water softener w/ fiberglass-body control unit?

We have a vintage-1971 water softener with an Autotrol 1550-TC (155 series) Fiberglass Control System.
The unit seems to operate properly in that it uses salt and the water is at least sometimes sufficiently soft. But sometimes the treated water is hard even right after regeneration. We are on city water from deep wells in northern Iowa. Sorry I don't have numbers, but I would say the raw water is pretty hard. The taste is not the best, but it is colorless, and I don't know that the raw water has any particular iron problem. Still, because it seems that the resin has never been changed or cleaned, my best guess so far is that the resin may be fouled with iron so that when the salt goes through the resin it doesn't stick to the resin and is wasted.
After googling some posts in this group, and since it's cheap and easy, I would like to try treating the resin with Iron Out or a similar product.
Will such an iron-remover bring any harm to the fiberglass body or seals or flapper valves or other parts of the control unit?
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