- Not sure about exposure time (for my lamp).
- Not sure about UV light position and height required.
With these you just have to experiment, I'd suggest putting the board fairly close to the lamp (20-50mm or so).
I suggest you take one longish strip of photoresist PCB and put something on top of it that blocks UV light, then expose the strip while moving the UV blocker away at predefined intervals (say, every 10 seconds). What you will end up with is a PCB exposed in steps for different exposure times (10s, 20s, 30s, 40s, 50s etc.), then just pick the exposure time that gives off the best result.
- Not sure if my mask pattern is opaque enough (to UV light).
If it's a normal transparency sheet it should be fine. Printing to these with a laser printer works but this depends on your printer. Large areas of black might get overexposed but this depends on your printer. If your quality of black is bad you can print two sheets and align them on top of eachother.
- Not sure about correct proportions of caustic soda to water
I have been using a 3% solution of NaOH (KOH works too) which seems to work well.
- Not sure about time to leave the board in the caustic soda - whether I'm going too far developing.
You just basically move it around in the solution until it seems developed (black stuff comes off the board), usually it develops quite quickly (under 30s) but this depends on your solution. If your solution is too strong or you develop it too long it will strip off all the photoresist if you keep it in the solution too long. A solution not concentrated enough won't successfully develop the board (again, 3% solution seems to work well). Washing the board under a faucet afterwards works well. You can continue developing after taking it away from the solution. You can actually even continue developing after etching the board a bit, just remember to wash the board. Putting the board in the etching solution shows you pretty well where the board has photoresist and where it doesn't, the copper exposed to the etching solution goes to this "dull" color/texture in under a minute in the etching solution.
- I also dont know if a developed pre-sensitized board stays so, and wont fade as its left in daylight over time. How long before normal daylight affects a pre-sensitized developed board?
A board with the plastic shield SHOULDNT be affected by daylight but to be sure I'd keep them in a place shielded from light (like a drawer).
Btw. I'd suggest you try to expose your boards with a 11W fluorescent table lamp (the ones with a "U"-shaped lamp, they should be pretty common and cost like 10e from Ikea). Put the lamp quite close to the board (like 50mm close, and have a thin plate of glass over the board to keep the mask close to the board). You can get suitable plate of glass from picture frames (again, Ikea is a good place to get these), just make sure its real glass and not plastic. Expose for 12-15min (I've used 13,5min for my boards and setup). I have been using this method succesfully for a long time. Won't work for large boards due to the lamp being so narrow but for small boards it works well.
The soldermask color is purely a function of designer preference, and whether the person paying for the boards is willing to foot the bill for custom soldermask colors. It has no relation to whether the board is ROHS compliant or anything else.
Green is the most common just because it's a defacto standard. You can get ROHS and non-ROHS green boards, as you can with any other coloring.
It's worth noting that it's entirely possible that that assembler only had an in-house standard where they ordered boards that needed to be ROHS compliant with a blue soldermask, and boards that didn't need to be ROHS with green (or any other color) soldermask. However, that would be a function of that assembler in particular, not an industry wide thing.
It looks like it might work "okay", certainly unlikely to damage your circuit in any way (unless sprayed in pots/switches/etc) Check if it says flammable in the datasheet to ascertain fire safety.
However, if you are just looking for a "sealant" to add after populating the PCB (as you mention in your question), then there are plenty of coatings around specifically for this purpose. They are usually know as Conformal Coatings.
There are various types and methods of application, so you need to decide which is best for you. Here is a comparison of the various types (acrylic, epoxy, etc)
Some will protect the PCB from moisture, oxidising, etc, but you can solder through them if you need to alter something (e.g. spray lacquers)
More permanent and hard wearing solutions include potting compounds (e.g. put circuit in suitable potting box, pour compound in, leave to set)
Look on places like Farnell, RS, Mouser for "PCB coating" or "Conformal Coating" and you will get plenty of options.
Here is an example of a spray on conformal coating you can solder through.
Conformal coating is not just to prevent oxidisation, it also helps prevent problems caused by contamination (e.g. acids/alkalis) or moisture (important for e.g. sensitive/high impedance circuits), and also can protect against arcing in high voltage circuits (with suitably high breakdown voltage rated compound)
If you are looking for something to apply before populating the board to stop trace oxidisation, then see the tinning suggestions in the other answers.