What in gods name is a "silancer"

Yeah they are threaded but i have no idea if it was only at the store I went to. Putting a small o-ring or rubber washer behind a nut on the schrader is what I do to seal it well (It can be done on both sides if you want"

Bike stores around my area have them

Engine enamel sounds fine.

You should look around a bit more I haven't seen any 2 inch sprinkler valves but I have a feeling they are cheaper than that since their 1" is almost 4 times as much as the one I bought after you convert currencies.

Degrease the aluminium first then sandpaper it to prepare to for painting. You'll have to do a bit more than that if you want a really good finish by using powder coating

Living in Queensland just south of Brisbane