Hi supa86, if the paint is too thick, it won't come out of the gun well, if at all, it could spit or skip and you may have to pull the trigger way back, losing fine control, or use a high pressure to get any flow. If it is too thin...well that's trickier. You can over reduce your paint (put a much higher ratio of water/reducer to paint) and still be able to use it fine. The pigment will be very reduced, so you will have to make more passes to get the colour you want, and you will need to be very careful with your trigger as it is very easy to let too much paint out at once and get spidering. But, if you have that contol, and keep the brush moving (which you should do anyway) it shouldn't be a problem. Working on paper, you may find that you are just making the paper too wet if you are having to do more passes on the same spot to build up colour, so less reducer would be good in that case. The best way is to start with with what comes out of the bottle, and then add your water/reducer drop by drop, until you get a good clean flow, and make a note of what you did, and what airpressure you had, and be aware that different colours don't always work the same.