Do boons have to be spent to trigger a weapon's critical, or is the weapon critical for free?

You have to spend them.

So, if the weapon has a CR of 3 and an ability is used that triggers a crtical for 2 boons, you can not trigger both if you have 5 boons. It is usually better to use the trigger on an ability, but you never know, it could be that your weapon has a CR of 1 due to a magical property.

Gobbo said:

If you've got the boons you can trigger any number of criticals in a single action.

monkeylite said:

Wow! so a weapon with a CR of 1 and a handfull of boons can be devistating!

You can trigger any number of criticals assuming each critical effect comes from a different source .

Your weapon's critical rating, an action card, special circumstance, ability, spell effect, or other game element may provide you opportunities to trigger a critical. However, each individual "side effect" (such as a boon or bane effect listed on a card or ability) can only be triggered once per check. ( WFRP p45: Effect Triggering Limits and p50: Side Effect Triggers).

You can't trigger the weapon's CR multiple times. Any single Boon effect can only be activated once. A weapon with a CR 1, for example, and *BBB* are rolled, cannot inflict 3 criticals by selecting the weapon's CR 3 times. The weapon's CR of one can be selected, inflicting one critical, leaving *BB* to other boon effects.

Edit: ynnen posted while I was typing. lol.