Musicalmunkie: Personally, I have found washing blankets with Nikwax and tossing them in the largest commercial washer I can find (at a laundromat) the best way to clean turnouts. I’ve never felt the need to do anything more; in my experience, that product gets them magically clean on its own. My gut feeling is that you are actually damaging the material by powerwashing it–twice. I mean, powerwashing will take the paint off a car…so I can only imagine what it would do to any type of fabric. I have used Wal-Mart waterproofing (as G&S mentioned), and much prefer Nikwax. It does a better job and does not damage the material like traditional detergents and waterproofing spray will. I have also owned front-loading washing machines and have never had an issue with them (poster G&S may have, but I’ve found it depends on what brand you buy and how it’s designed), although I would never wash a horse blanket in my personal machine. I ALWAYS take them to laundromats and use the biggest washer they have in order to ensure that I don’t damage anything (the machine OR the blankets). I’m not sure what’s happening with your blankets (I’ve never seen that before), but I would try doing as little as possible…skip the powerwashing, scrubbing, and anything potentially abrasive. The material isn’t made to withstand that kind of abuse.
Good luck! I’m a clean freak with horses and dogs, so I totally understand the struggle.
"Gentle in what you do; firm in how you do it." -Buck Brannaman