Out of my own ongoing curiosity, I emailed 3M the following question regarding tackiness:
If I am using Bondo® Fiberglass Resin, 401, and did not add enough
hardner and it is still tacky, can I apply heat to it to make it cure?
Their email response:
If not enough hardener was added the resin may never cure. Heat most likely will help the cure (again, if enough hardener was added). May take up to 3 to 4 hours to cure. If the product doesn't harden after 4 to 5 hours, the resin will need to be removed.