one thing i noticed that might be worth mentioning is that the developing solution i used had to be rather weak, at normal strength it stripped the resist straight off, not sure if this is the cheap board i was using or the way the light affected it...
Im using 10g per 1/2 litre of developer (sodium metasilicate)
I have used caustic soda for a developer for as long as i can remember, and the mixing strength is 7 grams per 1 litre of water so i think your mixing ratio might be a little to strong.
It is better to have the developer on the weak side, and if developing is to slow place the developer solution in a bucket of hot water to warm up, as it will develop faster the hotter the solution is.
I have seen a post from another person using the CFL light bulbs with good results to, which make me wonder what levels of UV are we being subjected to with the current trend of low energy light bulbs??????
I have had very good results using a CFL black light from Home Depot. i keep it approx 8 inches from the PCB and expose for around 7-8 min. I do use a old aluminum parabolic reflector I had on hand as well.
Caustic soda, is Sodium Hydroxide, the developer i am using is Sodium Metasilicate which is Sodium Hydroxide free, both are different chemicals.
Sodium hydroxide if too strong will strip all the photo sensitive layer off, best result i had using it was decent tracks but with lines through them breaking the tracks up
The climate is very similar to that in Northern Germany and I remember an extremely long summer one year.
It lasted from 12:30 to 15:00 on June 28 in the year of 1974.
Boncuk
P.S. I prefer UV-LEDs at a wave length of 340nm. They reduce exposure time drastically to a few seconds. Secondly I never use that cheapo paper stuff to make PCBs. Especially prototyping becomes a nightmare when desoldering a part ripping off the copper pad as well.