PCB design for outdoor use without coating
If your project is exposed to large variations in temperature and humidity, you may begin to see serious degradation in just a single season. It only takes a few degrees of difference between moist air temperature and your board to get moisture condensing on the board. Soldered/tinned connections degrade surprisingly quickly. 
If you want to test this out get a moisture detector PCB from Sparkfun and see how long it takes to corrode. Even in a non direct wet environment outside it's a problem after just a month.  

Potting or deep conformal coating is the only way to get real longevity (many years). 

Unless you are in a gland protected moisture proof box the ends of your cables (copper) will degrade over time. It's unlikely that you have significant current involved, but even at lower currents you cannot solder the ends of wires without long-term corrosion problems.

It is best to use solid copper cables and IDC or crimp the connections rather than use soldered or screw terminals (perhaps start reading here). 

there are plenty of options for IDC connections and these are particularly effective.
AVX have a broad range including SMT parts, and my personal favorite is Phoenix PCB IDC which does not require a pushdown tool and can handle both solid and multistrand wire. 

For conformal coatings you can't beat MG 422 IMO, but there may be other good ones out there. MG 422 is easily peeled back for repairs, which is why I like it.