Homebrew PCBs

Next, I need to get a conductive substance to plate the copper board
with, after etching.     Either immersion or wipe-on or electroplated.
 I am guessing wipe-on Ag powder would be the easiest.   

However, I must use *cheapest* rather than easiest or best.

I have 12 V gel cells, low-amperage DC wall warts, a low-amperage DC
dial-a-voltage regulator, a low-amperage +- 12 VDC power supply, and a
24VDC-7A power supply, in addition to regular 115VAC mains.

From surfing the web, I see that I can plate Cu with tin, silver, or
nickel.   Polyurethane is another "easy" option for a
non-chemically-bonded copper protectant, I guess.

So what based on your experience is the cheapest material that will do
a good job?

Alan