Hey everyone,
Elend: The reason it's not working well is because Tamiya acrylics aren't designed to be applied with a brush, just AB. I would recommend Vallejo acrylics for brush painting.
As for the ratio: I thin my Tamiya acrylics either 1 part paint to 4 parts lacquer thinner for a general coat or 1:6 for very fine detail painting. This might sound thin but if you need to apply more paint then you can just go over it again. It's better to thin the paint a bit more than to apply a coat which is to thick, you can always add more but removing paint without ruining the finish is impossible. As for isopropyl alcohol, it's a great acrylic killer, I use it to clean my airbrush. I tried using it as a thinner for Tamiya paints but I find it is harder to control because alcohol evaporates very quickly, making orange peel a much more common problem. Using lacquer thinner to thin Tamiya's acrylics means that I don't have to worry about the distance of my AB to the model when spraying, the finish is always smooth once dry. With alcohol as a thinner, it takes more experience and I've noticed that the finish never turned out as smooth for me.
Just my 2 euro-cents.