Washing insulated snowpants

You might want to pretreat the stains with spray n wash and a soft bristled brush then let em chill for a while before washing. Ideally you want to use the specialty tech wash stuff for that type of fabric, I think you can use powdered also but dissolve it in warm water first, don't sprinkle directly on to the fabric.

Have you seriously never washed your ski pants?

This is a pretty good guide: https://blisterreview.com/gear-101/how-to-care-for-your-waterproof-outerwear-down-jackets

Blíster has some other articles on tech fabrics and cleaning that should be helpful.

Using Nikwax Tech Wash is a great choice for outdoor sportswear, especially products with waterproof and insulating features. Nikwax not only cleans but also helps maintain the water resistance of the fabric without damaging the material.