Yes, same here, specifically regarding the paints-I haven't used any commercially-prepared washes, Tamiya's or anyone else's.

I undertook a hero's journey with Tamiya paints, to get where I am today with them.  When I started casting and painting toy soldiers back in the 90s, I bought paints as they were available, and for specific colors. I didn't mix any.  So I wound up with a good collection of Tamiya acrylics in my Farbkastl. And I applied them by hand, albeit over a primer.  I had the problems many report, especially that applying a second coat would lift off the first coat.

Then I got back into scale modeling, too.  I used rattlecans for applying main color coats, but still used Tamiya's acrylics, applied by hand. I got myself an airbrush and started teaching myself to use it.  And I read on some forum, that Tamiya's paints are formulated for airbrushing and so, are intended to be thinned for use.  I don't know if that's the company's official position, but it made sense to me, given my experience with the paints.

So then I experimented with different thinners, more because Tamiya's thinner struck me as expensive for the small volume, and my Dutchy senses wouldn't let me spend so much, if there were cheaper alternatives.  I tried water, because hey, "Acrylic means water-based!", right? (I learned that no, it doesn't).  Still got the clumping, the paint lifting off.  Then I tried isopropyl, because hey, Tamiya's acrylics are alcohol-based, right?  Well, alcohols comprise a range of organic compounds, and I think Tamiya's acrylics use some other alcohol as a base.  Isopropyl didn't work for me as a thinner for Tamiya acrylics.  I still got clumping, and paint lifting.  Though, it works very well to clean the brushes.

So I broke down, bought a bottle of Tamiya's proprietary thinner, and tried it.  Immediately, the problems with hand-brushing went away. Airbrushing it became simple and consistent, too.  I've gotten to the point where, with the paint properly thinned, I can lay down a coat on a piece, as thin as if I had airbrushed it. 

The only drawback for me, is that I like to use a wet palette for my acrylics, and I can't do this with Tamiya's acrylics.  So for hand-brushing, I use a small jar of the thinner.  I'll either dip the brush in the jar, then touch it to the inside of the jar lid to pick up the paint, swirl it gently, then apply it to the piece.  Or I pick up the color, then dip the brush in the thinner.  Either way has worked well for me.

I don't use Tamiya's paints exclusively; I have other acrylics, such as water-based Andrea, Vallejo, Lifecolor, and craft-store brands.  And I've got enamels, lacquers, and oils.  But having solved the Riddle of Tamiya, I didn't have to struggle on with them or discard them.

And when someone posts, "You can't paint Tamiya acrylics by hand", I can offer my own experience to explain how it can work.