Yeah the LEDs are pretty close to the board, about 5-7cm, I've noticed electrical tape gives a nice sharp edge that resists the developer very well.
At the moment I'm using an old scanner without the top, I tape one side of the artwork to the board and lay it directly over the LEDs (The LEDs are inside the scanner) then put a heavy object on top of the board.
I'm using Positiv 20 (postive) spray on photoresist which I dry with a hair dryer for about 5 minutes, it's pretty even when done so there is no real problem with uneven resist.
I'll try making the LED light more diffuse and use some black paper to prevent the UV being reflected, if that doesn't help I'll give sunlight a go.
Hi
Maybe I can help a bit
I use the exact same technique, with the Posetiv 20 and the LED's.
AND I had the same problem in the beginning..
I think your problem is the hair-dryer, when blowing hot air on the PCB, the resist flows to the edges, making the layer very very thin.
Try instead placing the PCB on a thin plate of wood (or enything else ;]), and heat it from the bottom with an iron to around 70c for about 10 min, after you sprayed it. (This is how I do)
I use 12g NaOH to 1 l water..
My LED array has a spacing of 7 mm centre to centre of the led's.
And I use an exposure time of 3 min, with a distance of 20 cm from the lamp to the PCB.
Find the right layer-thickness, and your good 95% of the time..
Hope this helps..
If you have any questions, please ask
-Mads