Hallo Mahmood,

according to your needs, there are several ways to implement a protection:

1. You could use solderlac from 'Cramolin', 'CRC industries', etc. It gives you some protection against oxidation for standard 'household applications'. Apply it after complete mounting, soldering and cleaning. But avoid contamination of connectors and sockets!!! It's hard to get these contacts clean and blank again!

2. Solderlac is only a simple protection against humudity and dust. Much better is the use of some plastic laquer consisting of acrylic resin. If I remember correctly solvent is pure alcohol, so nothing bad will happen to plastic materials of components. But again avoid contamination of connectors and sockets! In contrary to solderlac use of plastic laquer presents a problem, when you must do some later soldering work. Then, plastic laquer must be removed, somehow, e.g. mechanically.
I like this plastic laquer very much, because it works even under harsh conditions. In this case you should apply several thin layers on your PCB.
Plastic laquer is also available from 'Cramolin', CRC industries', etc.

3. If your PCB is exposed to real dirt and even water, use of conformal coating or encapsulation into elastomer should be planned. Here also several ways are possible:

a. Conformal coating with 1-component silicon rubber. A 1mm thick layer of silicon rubber can fully protect your circuit even under harshest conditions. Water absorption is absolute minimum. But keep in mind, that only non-corrosive silicons must be used! They come under names like 'oxime-' or 'alkoxy-' silicons. They don't use any acid components, only methanol- or ethanol-groups are built, which are non-corrosive. 'Loctite' has many good types: '5375', or the newer ones '5140' and '5145'. The latter type is specially developed for electronic use.
Keep in mind, that these silicon rubbers need natural air humidity to fully 'vulcanise'!

b. Encapsulation into elastomer, concretely spoken 2-component Polyurethan (PUR). 'Electrolube' has some nice systems, containing both components in one package, already prepared with correct mixing ratio. But keep in mind, that only soft, means elastic material should be used, when working with SMD parts, so that there's no resultant force on soldered parts after vulcanisation, which can be observed, when wide temperature cycles are to be expected. Something with Shore < A60 is soft enough for most situations.
Never use 2-component Epoxy resin. It normally vulcanises to a very hard material, not well suited for PCBs containing SMDs. Also, during vulcanisation many of them are becomming dangerous hot...

Good luck,
Kai