you can get flat clear, which will keep the original dull look, and it will seal the original paint, rust and patina.
you just have to make sure you clean everything really well (get any and all oil off of all parts to be cleared).
even the oil from your hands can make the clear not stick.
you want it clean of all oil before spraying the clear.
wd-40 will attract more dust & dirt than what's on the bike now.
I personally do not like wd-40 because it's not a lubricant, it's a water dispersant.