I got some AA16 cards from C.I.F at Farnell and got to work.
After reading a lot about various methods, I landed on using Sodium Metasilicate as the developer. Incidently, I had some laying around. More specifically, I had a 25g sample of the Mega Electronics universal developer, but after reading the datasheet I figured it is what I wanted.
For etching I used "Etching power for PCBs" from Scan Kemi that I bought last time I did any cards. It seems it is mostly sodium peroxidisulfate (sodium persulfate).
I tried various exposure times as well as developer strengths and development time. Three of them failed and three were closer to success
The failed ones:
1) 2 minutes of exposure, developed in a 13g/2.5dl solution. This was not long enough, the developer had a hard time doing much at all.
2) 3 minutes of exposure, developed in a 25g/2.5dl solution. I left the card in the developer for three minutes. When etching, the resist floated away.
3) 4 minutes of exposure, developed in a 13g/2.5dl solution. Some of the traces were too thin and broke.
The better ones:
4) 3.5 minutes of exposure, developed in a 13g/2.5dl solution for 5 minutes. When etching some of the tracks got too thin and some were not etched enough
5) 3 minutes of exposure, developed in a 13g/2.5dl solution. The development time was a bit on the short side so some resist still stuck and was etched away later. The result was probably the best one of all the trials.
6) 3 minutes of exposure, developed in a 25g/3.5dl solution for 1m 30sec. The solution was a bit warm as the additional water was hot when added. This turned out almost as well as the previous one but with slightly thinner traces and one short circuit.
My conclusion is that 3 minutes of exposure seems perfect, and that I should probably retry the 13g/2.5dl developer solution but leave the cards in a little longer.
The etching times was around 10 minutes, depending on the heat of the water. I did the etching in a small plastic box in a bath of hot water which started out at 44 degrees celcius. The recommended temperature is 50 degrees.