Getting the most from your paints takes some experimenting and practicing. As a painter progresses, they usually find themselves looking for ways to improve their painting and ultimately they come across someone saying, "Thin your paints, it's what you need to do."
You can search any of the big forums or lots of other blogs and most people say you want to get your paint the consistency of "milk." That's all good and well, but it doesn't always help. It's a great way to give you an idea of the general consistency you want to your paint, but it's not the whole picture either.
Let's look at thinning first and then some other things you can do to help.
So how much do you thin your paints?
For me, the amount of thinning depends on the brand and color of the paint. That means some colors don't need to be thinned and some do for my style of painting. The trick for me is to add only as much water as I need so that I do not create any surface texture (visible, textured brush strokes) when I apply my paint. This is created by paint that is too thick and your brush strokes end up showing on the model. Nothing kills a paint job quicker than this.
When I add water, the ratio varies all the time. I have no set amount I use. Most of the time though, I'll start with a tiny bit to see how the paint reacts and go from there adding more if I feel I need it. I don't use any fancy retarders or extenders when I paint. Maybe it shows, but most of the time, I need to move quick, so I'm shooting for tabletop results.
NOTE: Word on the street is that the new GW paints don't really need thinning. I haven't had a chance to use them myself, but I'd be interested in hearing what folks have to say on that.
Using the right amount of paint
This is a big one I find with folks starting out. You'd be surprised at how little paint you need to cover an area, especially if you add a tiny bit of water to it. If you're using too much paint to begin with, adding water isn't going to help.
Lower left, too much. Upper right, much better
Here's what surface texture looks like when you apply paint too heavy. You can end up with obvious brush strokes and unwanted surface texture. You can minimize this to a great extent by applying only as much paint as you need to in order to cover an area. Too much paint leaves excess on the model. This can be done before you even think about thinning with water or any other extenders.
Another way to avoid surface texture is to always keep a wet edge to your paint. That means not letting the edge dry out as you paint across and object. Larger objects make this tougher to do, but smaller things should not pose a problem. If you let the edge dry, it has the tendency to leave a tiny amount of texture in the form of a lip where the first layer of paint dried and where you go back over it with another pass to finish off the area.
Think of the "tide marks" a wash will leave when it dries... it's like that but with a raised edge. You don't want that.
Priming and the color you use matters
The reason your primer color matters is because it helps fill in the minor places you have color imperfections. Light colors over light primers and dark colors over dark primers. If you know that adding a bit of water to your color will make it ever so slightly transparent, then it helps to have a similar color underneath to support it.
If you look at how the different colors act over black and white, you can see some will need less layers of paint to get a nice, smooth finish depending on what's under them. The light tan color will end up better over white than black. The purple color will do better over the black. If we know we are going to thin to help reduce any surface texture and thinning makes our paints slightly transparent (even more than they may already be when they started), why not make it as easy on yourself as you can?
Multiple layers of slightly thinned colors is better
Using good priming practices (light for light, dark for dark), we are better off trying to get a good, smooth finish to our areas by using a few passes of slightly thinned color than one heavy pass. This approach allows us to keep more detail on the model and we run less of a risk obscuring it with thick paint.
What about too much water?
If you use too much water, you're basically turning your paint into a wash and it will act accordingly. Pooling in the recessed areas, allowing the color underneath to show through, tide marks and so on. This can be good or bad depending on the look you want.
Most of the time, this is bad because the person wants the thinned paint to minimize the chance of surface texture or the buildup of paint, but finds themselves adding a hundred layers in order to cover up what's underneath to get a smooth, consistent finish to that area of their model... which results in built up surface texture and a large build up of paint in the end.
Be extra careful when washing over light colors such as whites and yellows or any light color for that matter. Since they are so light, the wash picks up (makes visible) all of the surface imperfections. This can quickly make your model look horrible as you can see every little unwanted surface texture on the model.
Where do you start then? Here are a few things I do:
1. Clean your models beforehand to remove any chemicals, films, grime you can't see. Nothing fancy, just a quick wash with warm soap and water after being assembled.
2. Don't handle your models when painting them. Mount them onto something to avoid getting skin oils on the model which will not react well with paints and prevent you from getting that smooth coverage. Think trying to paint Forge World models without washing them first.
3. Shake/mix your paints each time before you use them.
4. Prime with an appropriate color to help "hide" inconsistencies.
5. Add to the previous one the idea of using similar color base colors. For example, Ork skin, a dark green base color will help you with applying you brighter greens and getting a nice finish. GW Foundation colors are perfect for this kind of thing.
6. Make sure you don't overload the brush for the size area you're painting. If I know I'll need more paint than what I can put on the brush in one pass, I'll transfer my paint over to a small palette and work from there. That way I can mix my paint and water, clean my brush off quickly and then put the right amount of paint back onto the brush before I touch the model.
7. Thin with just enough water. I get some paint on my brush and then dip that into my water pot. Since I have my brush only covered about 1/3 of the way up the bristles with paint at any one time, I dip only the tip into the water. Some paint comes off and some water is picked up by the brush at the same time.
I roll my brush on my holding base (the thing you mount your model to so you're not holding it with your fingers while trying to paint) to mix and remove excess and then go from there. This can also help reshape the tip of your brush into a fine point.
This is what the caps (I use regular spray paint caps) I stand my minis on look like after a few paintings. You can see all the little markings along the edge where I have removed excess paint from my brush before painting. Different folks use different things to do this.
All in all, thinning your paints will help with getting a smooth application on your models. But there are other things you need to be doing as well to help that process along.
Make sure to check out these posts as they might help:
The Back to Basis series
Priming with colors other than black and white