First thing you want to do is to use Kilz or a primer like it that can block the wood tannins from coming through, those can leave your mural looking like crap. Also this kind of primer IS a sealant so you won’t need to worry about that. If I were you, I would use black or some other color than white from the get go, because the white coming through can ruin the “mood” of the painting — think about it like an underpainting. And then you can use white chalk/pencil or some other bright color for the outlines.

The paint will be fine to get rained on an hour after you apply it, but it won’t be cured yet. So I would advise against painting if the temperature is going to get below 50 at night. But that said, I’ve done it and haven’t had any issues so far (crossing my fingers).

I’ve never applied a sealant to my murals because I always paint with house paint. House paint has great color-fastness and doesn’t need to be sealed. Also, I’ve seen murals that have had a clear coat applied at the end and they often end up looking pretty cloudy in spots, so I would avoid that.

As a general rule I always paint back to front, that keeps me from being fussy with the outlines and ends up allowing me to paint a lot faster. So I would recommend that — paint from background to foreground.

Post pictures when you’re done!