If you google Hoya filter problems, then you will find some complaints about difficulty cleaning their filters. I'm not sure whether it's true, I have not used Hoya filters before. I have been using B+W filters for years without problem.
To clean my lens and filters, usually I just use an air bulb with brush to clean off visible dust, then use my breath to fog up the lens, and use a lens paper tissue to clean it. I haven't had problem cleaning any lens or filters this way in the past. The lens cleaning solution and microfiber sometimes leave streaks and rarely dust behind. Make sure you keep your microfiber cloth clean and free of dust. Lens tissue paper does not have that problem, I only use it once and discard it after use.
I just so happened to open my brand new 72mm Hoya UV filter for my lens at a time when the sun was shining through my window, and I noticed these little "particles" all over the filter. I tried cleaning it with a microfiber cloth with no luck. You can't see the particles when not holding up to the sunlight. Are they normal??
Thanks in advance for advice.