I think we all hate how oxidized homemade PCB's get over time, and I also think we all love how easier it gets to solder SMD parts when you have a soldermask.
I myself tried to find a source for low quantities of soldermasking ink, better if photosensitive or UV curable, but I was unsuccesful. All I could find were buckets of the pro stuff for â‚¬â‚¬â‚¬,â‚¬â‚¬ in strange distributors.
Until I found this: http://cgi.ebay.com/UV-Curable-Solder-M ... 2eb527251d
I found it like a year ago, and I thought I had to give it a try. Bough a syringe and... my life has changed since then. Once you know how to use this stuff properly, your homemade PCB's will never look the same.
It's a bit hard to learn how to use that stuff, however.
The recommended way to apply it is to squeeze some drops in the centre of the PCB, cover it with a plastic film and extend the product with a squeegee all over the board forming a thin layer. But there is a problem: this stuff sticks to nylon, acetate and polyamides, so you can't use transparency films with the photolite already printed on them, because you'll mess up the board, so I'll tell you how I apply it.
The first thing you must find is a sheet of any thin but still quite rigid plastic. It must be as thin as possible but it must NOT form wrinkles, and be as smooth as possible. And it also must not stick to the stuff. Seems easy to find such plastics, but it isn't. I get mine from paper sheet packages. When you buy 500 sheet packs, they usually come wrapped in a kind of plastic that is just perfect for this job. You have to remove the prints from the plastic, I do it cleaning the plastic sheet with common ethil alcohol, until it's totally clear and transparent. Don't worry, plastic sheets are reusable and you won't have to buy a new 500 sheet pack every time you want to make a board.
Then, I apply a few drops of the product to the board, I put the plastic sheet on top and I extend the product all over the board with a plastic squeegee . The layer of product must be thin. If it looks too green then it's probably too thick. Once I have all the board covered with the product as uniformly as I can (a sandwich of board, layer of product and plastic sheet), I place the transparency film with the mask printed on top of the stack, and I align it with the board. Then I sandwich everything between two layers of glass and insolate the board for about 30 minutes with a 8 watt blacklight tube. Once it's done, the stuff that has been exposed to UV light will be hardened and stuck to the board, while the stuff that was covered by the mask will still be liquid. I peel off the plastic sheet that shouldn't be stuck to the ink (the plastic sheet shoudn't have rests of hardened ink attached), and the liquid ink that remains in the board can be easily removed wiping it with a towel and some alcohol or something. Then I clean the plastic sheet (which will have some rests of liquid ink) so it can be reused.
The results are pretty amazing, achieving very very high resolution masks that are really useful to protect the boards and facilitate soldering. Usually the coating won't be as uniform as one would want, and the texture and appearance of this solder mask isn't the same of an industry applied mask, but does its job perfectly.
I'll post pictures of the process if requested, but here's a sample of some homemade boards I did with the stuff applied: http://lulzimg.com/i22/47ea9d.jpg
(I took the same image I used for my presentation thread, obviously the OLS and the BP boards aren't homemade :P) I don't have other pics right now. The robot you see has three LGA or QFN -packaged IC's (a buck voltage regulator, a dual H-bridge and an electronic potentiometer) and the mask was perfect for them.