I have this same problem in oil and gouache, but it is worse, I think, in acrylic. Let’s say I want to paint a light green blade of grass against a dark green background. If I thin the paint so that it flows nicely from my fine tipped brush, the green stroke is too transparent. If I make sure the paint is thick enough to be opaque, it makes a blobby thick stroke that is not the right elegant grass blade shape.
Tips? Tricks? Thank you.
Add a little white to your paint then after the acrylic is dry, paint without the white. Or make a mix of paint to get the color you want. It will be more opaque. Acrylic is my go-to medium and all areas of my paintings are at least 3 to 10 layers. Keep in mind that acrylic paint shrinks so layering is pretty much a must.
Thanks Melinda. I don’t quite understand your first sentence - add white and then paint without the white…?
I do understand the rest of it and I have tried it with mixed success. (no pun intended). Another example I thought of is painting a cat whisker. I defy any artist to reveal the mystery of painting a perfect white thin cat whisker in one stroke!
Sorry for the confusion. What I meant was add a little white to the color you’re using on the first layer of paint. It will give you the opacity you need. Then after it dries, paint over it with the actual color you want.
As for the fine line - I do that all the time. You paint the line first and after it dries, paint the color next to it to straighten it up and repeat on the other side of the fine line. You may need to touch up the fine line and repeat the colors next to it. Maybe a few times. But you can get a fine line that way. I use the technique for lettering.
Yes, what Melinda for opaques in acrylics. It is in the building of layers.
Also for fine lines in addition to painting up to a line with the adjacent color, to refine or thin out the line, as she said it really is about the brush, the tool. Use a good quality brush and one reserved only for creating those fine lines. Don’t use it for anything else.
It does not have to be an expensive brush, medium grade will do, but NEW and again save it for fine lines, nothing else.
The next thing is the viscosity of the paint needs to be just right for that balance between flow and opacity. It takes a little experimenting to learn how to do it.
Some like rigger brushes for fine lines. They are okay but I find them a bit too flimsy for control. A liner brush is better because it is shorter (better control) than a rigger but is still thin, comes to a good point and still holds enough paint.
I will use a good flat brush on edge or a round that has a good point.
Another good brush for fine lines is a ‘Cats Tongue’. It comes to a nice sharp point but holds a lot of paint for those longer lines.
For fine lines, use Acryla Gouache. It’s basically opaque acrylic. Then if it looks too flat, glaze over it with acrylic thinned with matte medium.
Thanks Connie and everyone. I’ll try the suggestions and see what happens!
You can use a lettering brush AKA script liner brush size 0 to make a thin stroke like a cat’s whisker. The paint has to be thinned a little to flow well-- or you can use liquid acrylics. It does have to be just the right viscosity.
Re. layering. I do that, too, add white paint to whatever color in the first layer, and then the pure color over that.
For really fine acrylic lines, put paint in an old style metal ruling pen. Paint consistency like thin cream. 
You could try using a slow dry medium, it will thin the paint somewhat but wont make it overly opaque. You could also try using high flow acrylics if you like thinner paint consistency.