I'm etching a double sided circuit board for my project in electrical engineering but somehow the PCB gets these weird marks and turns out bad. I tried different things like washing the PCB before hand, being really careful not to touch it etc. but it still turns out like this. And it's usually one side.
The process I use is to have regular printer paper, front and back on two A4 pages then tape them together around the PCB. UV light for 300 seconds and then remove the film with NaOH. At this point the PCB looks OK but it gets these marks. I then wash it carefully in water before I etch it. I'm baffled what happens here, does anyone have a good answer for what I am doing wrong?
Edit: The problem occurs before etching. When applying NaOH the PCB gets weird marks that kind of resembles water drying on a surface. The UV light is a dedicated machine with UV light tubes that is tested with these PCBs to work on 280-300s with normal paper at my university. It's highly unlikely this is the source of the issue.