Print it, Iron it, Etch it, Mask it, Drill it...

[davent]

Amazon.ca has a 14g block of violin rosin for $1.79.

[micromegas]

My favorite PCB coat, which i have been using basically since i started making my own pcbs  is rosin dissolved in acetone/IPA. When dried it makes a thin flux film all over the board, stops the oxidation and makes the solder joints nice and shiny. Some of the boards spent like 15 years in basement, still looking good. The trick is to use a solvent that evaporates not too quickly (you'll have enough time to paint the pcb) and not too slow (the board will by sticky for a very long time). Acetone does a great job for me. I just buy the rosin in pieces, crush it into powder ,dissolve it in acetone and paint the pcb after drilling. Usually i let them dry overnight. Really recommend that method, it makes the soldering and repairs much easier (there is no such thing as too much flux!). Here is one example:

I've tried something similar: violin rosin disolved in acetone, but didn't crush the rosin into powder. The main problem was that my mix was too sticky and dust particles adhered to the pcb.