I guess I don’t worry too much about washing. I rarely leave Toby unattended in his woolie (like Fred, it’s an old school Newmarket striped rug. Very thick and heavy). Occasionally I’ll use it as “underwear” for really cold temps, but mostly he wears it while I’m grooming him or he’s cooking in his massage pad. And he’ll wear it after a ride if I’m fussing with him in the cross ties (normally, he’d wear it because he’s sweaty and needs to dry off, but since we’re most just walking and doing very light trot and a wee bit of canter, and it’s COLD, we’re not breaking a sweat!). He wears it in the trailer for cold weather shipping, too.
I also will layer an antisweat under it if he’s had a bath. Yes. Occasionally, he gets stuck in his stall to dry or hang out and will have a roll, but shavings brush off the wool easier than fleece. It just doesn’t get that dirty the way I use it. I might wash it once a year. This year I may have the blanket lady clean it for me, as it does need a minor repair.
I do love the Thermatex, though. Yummy stuff. I have a Smartpak knock off. I think they call it Thermo Manager. It isn’t nearly as heavy as a true Thermatex, but it is nice and wicks like crazy.
I don’t typically use walking coolers since I clip and they are never so hot they need a walk to cool out (like a race horse would). I did have the Dover one when we were swimming horses a couple of times a week one winter, and it was quite nice for the price. I usually put an antisweat under it, and a wool or fleece over it for the trailer ride home. They were always toasty and dry by the time we unloaded.