One important factor is the expected life or reliability. Are you trying to make a product or just a hobby project?
If a hobby project, you need to seal it well enough it will last a reasonable amount of time and it doesn't fail shortly after starting to use it.
Not sure what switch you have in mind so I'll put that aside for now. The LED can be purchased as "waterproof" in eBay. I would expect these to be at most water resistance but will likely work for some time.
For the PCB. One of the issues with conformal coatings is they don't cover sharp edges very well. And can be this in areas allowing moisture to eventually cheep in.
I would suggest a piece of heatshrink and an electronics grade silicone RTV. For hobby use I know you can't use the type that emits acetic acid when curing. You might be able to use the newer grades that don't smell as bad like GE Silicone II. I'm not sure what their curing byproducts are.
You should use an electronics grade (i.e. non corroding) of RTV. You can find them on Amazon, Digikey and likely other outlets.
For a trial you also might try hot melt glue. If you use the silicone wire I mention below you will get much better adhesion from the potting to the wire if you use a silicone sealant (aka potting compound).
The procedure I've used:
- Coat the board and wires with sealant, lots of sealant, you want not to have any air trapped on the board
- slide the heatshring over the board and sealant
- Shrink the heatshring
- allow to cure
A suggestion on wiring. If you want to be able to replace the LED and/or switch after the PCB is potted, I would simply run the wires up the arm to a relatively dry area and make the connections there.
Another word on wiring. I've recently found some neat very flexible silicone wire. It may be useful for your project.
ebay silicone wire
There are likely other vendors, but I only have wire from this vendor and am very happy with it.
Again if you are trying to design a saleable product none of the above will be adequate if you expect any reasonable kind of life.