anyone know where I can find patterns for the whetstones needed to craft weapon repair kit?

There are some traders (like smithies, armorers) that will sell whetstones (elven, dwarven, etc.).
Have not seen any diagrams for whetstones.

Really not worth it (at least in versions before 1.10). The cost of making the kits was more than the cost of buying them ready made... and more than the cost of visiting a smith.

I collected the diagrams for completeness, but as with most diagrams, never bothered using them.

There have been some changes to the economy, and with 1.11 it might be marginally viable (not checked to be honest), but I still find smithy repairs to be as good as anything, and for the wilds, the drop rate of kits exceeds my usage of them and I end up selling most to keep weight under limits, rather than using them.

The total effect of damage is around a 10% malus (times your weapon skill bonuses), while the benefit of oils varies between 10 and 50% (flat values), and grindstones and tables 20% (flat value).

If you are very skilled, and use lots of mutagens, then the impact of damage can overwhelm the benefits of oils and stones, but for most hybrid builds the average effect is probably a wash.

Worth using.. yes.
Worth making? IMO not so much.