How do I slightly stiffen a yarn feather?

Yes for sure! A little watered down glue or Mod Podge will work. I would wet it first to help even with absorption of the water/glue mixture and then apply the mixture. The more water you add the softer it will be when it hardens.

Hello,

Yes either would work.

I like Modge Podge for all my decoupage and crafting needs It comes in several sheens gloss matte and satin, and application finishes to include fabric, outdoor, glitter,dishwasher safe, pearl and more!

https://plaidonline.com/products/brand/modpodge?count=80

https://modpodgerocksblog.com/learn-how-to-mod-podge/

They also sell stiffening spray - not too expensive. Craft stores should have it as well as Amazon.

Karen is right, she's talking about sizing, it's a spray starch sold in the isle where you find laundry soap

Yah, starch would work great.

What about old-fashioned starch?