Will a white painted bike get rusty when stored outside?
The problem with sealing it tight is that water will get in, even just as humid air, then it can't easily get out. You'll get condensation, which will cause rust.

It's much better to shelter the bike so that rain can't hit it or blow in easily, but leave the bottom open so that water than run out. A motorbike rain cover would be ideal, but a tarp thrown over with a rope looped around lower down to hold it in place will be fine. Try to cover the rear derailleur and the chain when doing this, as those are especially likely to rust.



Note that it's a really bad idea to spray the chain (or anything else) with a degreaser like WD40 before you do this. You're better off buying proper chain lube and over-lubricating the chain and derailleurs before storing the bike for any length of time. That excess will better protect them, and then when you pull it out of storage add a little more lube, spin the pedals backwards 10-20 turns, then wipe the excess off (that replaces any volatile parts of the lube that have evaporated while it was stored)