Hi Richie,
The short answer is 'Yes', the sun is actually a good tool for doing this job. However, the UV index for the day should be quite high and you should try to face the board directly to the sun.
You will need to experiment with exposure times, which will be variable everytime. This is why buying the UV light is better, because you can predict exactly how long it will take to get the job done. The sun is a good source though, since it is fairly collimated (2 degrees)..
I am actually working on a secret exposure unit and will be done in a few months.. I will let everyone know after I decide whether or not to get it patented.
Regards,
Stephen