Chris..........you are taking a big chance here. Most paints say right on the can - do not thin. Use the correct sprayer.
Apart from effecting how the paint dries, the sheen, and the color - durability and mildew resistance suffer greatly when paint is thinned.
Think about it - a properly applied - non thinned paint film is about the thickness of a piece of paper/coat.
Now if you thin that - you have an even thinner film - how long do you expect it to last? I also will tell you that if you do have a failure - it is very easy for most manufacturers to tell how thick the paint film is. A very thin paint film will leave you no recourse should it fail - you will be on the hook for the repaint.
Use the correct tools for the job - it is worth it in the long run.