Jumping in on this one because I use the Liquitex and W&N products you just mentioned.
I use almost exclusively Liquitex Slo-Sri Fluid Retarder for wet blending techniques. A lot of folks here don't really wet-blend but layer, so you're talking two completely different techniques.
Acrylic retarder extends the drying time of paint without reducing opacity. But be careful, too much and it will reduce the strength of the paint - the stuff will flake off.
Flow Improver helps your paint go farther again without reducing opacity.
For classic wet-blending, I paint a base color allow to dry. Then I mix up three different colors. The base color, a shading color, and a highlight color. Depending on exactly what colors I'm using, I may just darken or lighten my base color, or I may use black for shading and white for highlighting. Anyway, I mix these with the Liquitex Fluid Retarder. I usually start shading first, so I paint a deep recess with my shadow then start pulling it out of the recess. As I come outward, I'll mix my base color into it until I get a nice blend and the dark is where it's supposed to be, not out too far. This is just going to take practice - trial and error - for you to get the hang of it. When I do the highlights, I'll hit the highest point with my highlight color and start pulling it down then again start blending with the base color. Typically darker colors are stronger than light so you'll want to go sparingly otherwise you end up with a muck gray that never dries.
I also keep spare clean brushes on hand to help feather and blend so I'm not adding a lot of paint.
Liquitex Slo-Dri Blending Fluid is NOT the same. It's glossy for one thing and I find it actually thickens my paint. I really don't care for it but since it has a bit of gloss, I might sometimes mix it with my metallic paints.
Liquitex Matte Medium I use when I'm making ink washes. One of the tricks to get the ink washes into the recesses is to add Future Acrylic Floor Polish (If you're in the UK it has a different name). I'm in the US and I use Mop n Glo. Same stuff basically. But this stuff also adds shine. So, I add the Liquitex Matte Medium to help kill the shine. I also have a particular blue ink that whenever I apply anything over it, even when it's dry, it starts bleeding and comes to the surface. Drives me nuts. I mixed the Matte Medium with it and while it doesn't stop it entirely, it helps a lot.
One time my store was fresh out of the Liqutex stuff I usually get so I got the W&N fluid retarder and the Flow Improver. YUK! I personally can't stand them. They didn't work well for me at all. Next time I hold out for the Liquitex. lol!
Anyway, that's been my experience with those products. I hope it helps!
