RosaA4780
I seem to be having a bit of trouble with my paint being runny, and not adhering well to the surface of my models at times I am using tamiya paints.
Tamiya acrylic paints (I'm assuming you're using acrylics) can be a little tricky to brush-paint.
Firstly, you say the paint is runny? That's a little unusual for Tamiya. Perhaps you haven't mixed the paint thoroughly. For best results, stir the paint for a few minutes until there's no obvious lumps and/or strings. Paint settles in the jar and after some time on the display rack at your hobby shop, it takes more than a few shakes to break it loose.
Once you're ready to paint, be aware that Tamiya paints have an annoying tendency to "pull up" when you brush back over an area which has previously been covered.
Brush back only as far as the wet edge, do not attempt to brush back over an area to try to smooth it out. It will pull up the previously applied paint in big ugly lumps.
Thinning your paint with one part thinner to three parts paint. This will help it flow and reduce the tendency to pull up.
Don't be tempted to recoat too quickly. Apply a coat and then allow 24 hours for the paint to cure before recoating.