handcrafted PCB problems
It's hard to get the process to work if you have many variables creating uncertainty:


  exposed it outside for 25 minutes for each side (cloudy winter day in Europe)  


Use a consistent light source, and experiment with different exposure times. I mount a cheap 10W 395nm LED COB 20cm from the PCB, and expose for 30 seconds per side. 


  Applied negative photoresist paint


Are you sure that the paint is applied evenly and without defects? Is the thickness repeatable? Dry film photoresist is easier to apply. Presensitized boards are even better.


  etched with an unknown ratio solution of HCl + H2O2


The peroxide decomposes on its own, as you have noticed. I use ferric chloride, as it's still usable after months.




  tracing paper  


I've tried that too. It works, but the edges lack definition compared to transparencies.