I would agree that transparency should be used at least until the process is working OK. The mottled effect seems to be common to a number of your photos, suggesting there may not be enough contrast (to UV) between the paper and print)
There should be no photoresist on the non track parts, otherwise you end up with areas no etching, then the tracks being etched when you extend the time to try and etch those stubborn parts.. I've never tried rubbing it lightly as someone suggested, but it may well help.
When you develop, can you see the photoresist floating away from the areas to be etched? On mine, I see the tracks appear, then the photoresist floating off, as two distinct stages.
20C isn't ideal, but it shouldn't cause the problem in the photo.
There should be no photoresist on the non track parts, otherwise you end up with areas no etching, then the tracks being etched when you extend the time to try and etch those stubborn parts.. I've never tried rubbing it lightly as someone suggested, but it may well help.
When you develop, can you see the photoresist floating away from the areas to be etched? On mine, I see the tracks appear, then the photoresist floating off, as two distinct stages.
20C isn't ideal, but it shouldn't cause the problem in the photo.