Currently. I'm making a PCB using the photolithographic method (print on transparency, expose with UV which works, then develop under a bright red LED as the only light source), but half the time, the developing stage is not perfect. I find this out when etching the board is about 5% complete. I then re-develop the board. This is even after I sanded the photo-resist layer off of a small corner of the board (before exposure) to give me an idea of how the background should be after the removal of the unwanted photo-resist.
I'm using 1 part standard sodium hydroxide developer to about 10 parts water and the liquid is greenish since I use the same mix for multiple boards, and it takes two minutes to development to (roughly) complete, but I cannot use time as a factor to development completion.
Rather than having to re-develop the board to get it perfect, what other signs should I look for on the board to indicate completion of photo-resist removal? and would it be a benefit if I used a different colour light source to make the signs I need to look for more visible?
I want to avoid colours which would make the photoresist react (such as the exposure light color purple).