Even tinned, the PCB traces will still corrode. Soldered joints will also oxidize since the solder is exposed to air.
After soldering, the board could be further protected by applying a coat of clear lacquer. The lacquer dries fast and can be soldered through afterwards if needed.
I use this lacquer for final board protection:
I found it at a local Hobby Lobby store. It dries in 20 minutes and gives that “wet” and polished look. I applied it after soldering of most of the components, but before soldering connectors, push-buttons and some other components. I coated the top side after putting SMD components, then soldered a few through-hole parts (terminals etc.) and then coated the back side. I protected SAM3 and SMD LED’s by sticking on small cuttings of painter’s blue tape over the components before spraying it over.
All the soldering done after this lacquer was applied did not affect the quality of the joints in any way. Later soldering of workarounds and corrective jumpers over it definitely evaporated some of the protective coating at specific spots, but those places are not obviously visible and it still looked and worked well.
Spray outside and follow the instructions: don’t spray directly at a board but use a sweeping motion. This unfortunately appears to waste a lot of spray as you are passing in and out across the board.
Tools: Testors wet look clear coat lacquer, nitrile gloves