Welcome to Librarium Online!
I've been working on a WFB Dark Elf army for some time now, and while the better part of my army looks fairly decent in my eyes (to me and my shaky hands, table grade is quite lovely), but there is one area of painting in which I've found myself confounded time and again: Wide open spaces. In particular, I've found the issue in painting skintones and the wider swathes of outfitting (the tunics of the Glade Riders I'm kit-bashing into Dark Riders come to mind as well as the skin of my harpies and witch elves).
When working with these wider surface areas, I've been running into two quirks in particular. The first is the increased visibility in brushmarks in the paint. I've tried thinning the paint some, but adding even one more drop of water stretches it to a point where it becomes blotchy at best. I've also tried simply adding a fresh coat over it, but that simply adds a new layer of brush marks to replace the old ones.
The other oddity I've found (with flesh tones amusingly enough) is a tendancy for 'goosebumps' in the coat. The coat will be smooth without the brushtrails mentioned above, but instead has tons of small nubs across the area, giving the model a somewhat fuzzy look (and no one wants a witch elf with fuzzy legs...). At first I thought it may just be the paint (I was using a cheaper Folk Art paint), but after a trip to the local hobby shop, I found that even with a higher-end skin tone (Vellajo and Reaper), I was still getting that oddly studded look to the models' skin.
Welcome to LO.
What brush are you using? Sounds like you got some old ones.
Some general tips:
1. Shake your paint cans before use. 10-15 seconds, especially important for Vallejo paints.
2. Get yourself a proper pallet or use a clean piece of plastic (like an old lid or something. I use the back of old Rackham blister packs).
3. Brushmarks are a result of using too much paint in one go and not diluting it enough. My suggestion would be to try to use smaller amounts of paint and keeping your brush moist. Mixing in some water in a drop of paint should do the trick, but add too much and your paints will be to runny.
I hope you'll be able to solve the problems. Any chance of you getting some pics of your models while you're at it?
I'll see what I can manage. Admittedly they'd be from my iPhone, so I can't guarantee getting anything decent clarity-wise, but I'll see what I can manage.
Here we are, admittedly, the quality on the pictures is far from spectacular, but it does show some of what I'm talking about.