If you're finding the paint goes on 'chalky' even after thinning with water, flow release is your best friend. It helps break the paint up in the medium, so it flows better off the brush and dries smoother. I find white and flesh paints particularly prone to this, so I use a water / flow improver (20%) mix to thin the paint a little to get thin, smooth layers. You need several coats (waiting for each to dry before doing another) but it gives good coverage without obscuring detail or leaving bumps. Don't use too much flow improver though, or it becomes very hard to control and doesn't stay where you put it! If you are struggling to control it, a bit of matte medium will thicken the paint physically, while leaving the colour alone.
One trick I'm going to try out is borrowed from tank painters, a pin wash. Basically, after doing the white coat shading/highlight that you want (or just a flat white base coat if you're not trying to be fancy) do a gloss varnish coat (spray or brush). This protects the white, and makes the surface slick. You then use the black wash on the lines and crevices specifically with the tip of your brush, rather than slop it all over. Importantly, you then mop up excess wash with a clean brush and/or wipe it off as you go, leaving the flat white surfaces white, so you don't have to do much - or ideally, any - new white paint to cover up the wash overflow.
The wash needs to stay wet long enough to not grip on the surface where you don't want it, as well as flow nicely into the crevices, which is why many tank painters use oil or clay based washes as it's easier to control and clean off than an acrylic wash. But then you end up needing turpentine to clean your brushes, which is no fun. So an acrylic wash will also work, but a little bit of additives can help. You can thin GW washes with a bit of water + flow improver, which will help it get into the crevices easier (and easier to wipe off). Adding a bit of matte medium will extend the drying time a bit, and also help prevent 'tide rings' where it dries too quickly around the rim of the crevice rather than at the bottom, and will help thicken up the thinned wash so it's easier to control. GW's lahmian medium is an (expensive) alternative that achieves much the same aim if you don't have easy access to an art supply store. GW's painting videos almost invariably thins every wash with lahmian medium first.
Again, don't overdo it on the thinning/mediums, as if you make it too thin it becomes a glaze rather than a wash and won't 'settle' in the cracks, which is not the effect we're after.
In short; flow improver and matte medium make tricky paints much easier to use, even if they take a bit of practise to get the hang of the exact impact a given amount will have. My painting improved considerably when I started using them.