I actually almost gave a history of WD40 and its predecessors and their use at the Atlas missile sites (I'm a big rocket nerd) and how tins of the stuff kept "dissapearing" home with the technicians.

For those curious, its mixture of hydrocarbons designed to spread out over a wide surface (which is why it also penetrates into cracks and small spaces) and then the lighter hydrocarbons evaporate off, leaving a protective, greasy-feeling film. Unlike a dediated lubricant though, the film is made of short chains of hydrocarbons which make it a poor lubricant - the film is too thin, and the hydrocarbon chains too short, so if you use it as a lubricant, it just breaks down and stops lubricating, and then cements together with metal particles to form an abrasive paste. Yum!

Being a mixture of hydrocarbons it is an organic, nonpolar solvent (which is why it doesn't mix well with water, a polar solvent), which means it can displace and dissolve grease, at the same time as repelling water. I use it as the first-wash degreaser on my chain (then I use a water-based cirtrus degreaser to get the WD40 off and give it a deeper degreasing, and then finally it's good for oiling). It does have the capacity to attack some paints, but it was designed with that in mind and most cured paints are resistant to it. The stencilled markings on the Atlas, for instance, wouldn't dissolve when you gave it a spray-down. You should still do a test in an unobtrusive place high on the frame to make sure though!



Back to rinsing the tubes/frame! I'm inclined to agree with [MENTION=469965]Koyote[/MENTION] - I shall revise my recommendation; as they said, you should already have rinsed all the important bits!

If we run through all the tubes, we see that we have open tubes and closed, or mostly-closed tubes.

The open tubes (front tube, seatpost, bottom bracket) have the most exposure to air and the elements, so they're the worst spots for potential corrosion, but those are also the bits you've cleaned

The mostly-closed tubes (top tube, downtube, seatstay, chainstay), in an ideal world, would be entirely sealed. In our less-than-ideal world, between voids in the welding, bolt holes, vents, and internal cable routing, you;ve almost certainly had water in it, but you would have a hard time rinsing it out unless you were very patient and had a very small tube to inject water into it - and then you have to get all that injected water back out!

My revised advise would be to squirt WD40 in wherever you can and give it a swish around - which you can do without dissassembling the bike (I do this every 6-12 months as I'm out in the rain a lot and theres a lot of condensation in the cold weather)

What I do is stand to the side of the bike, squirt an excess of WD40 into the tube-of-interest, rotate the bike lengthwise until the tube-of-interest is horizontal, lift the bike off the ground, and finally pump my arms back and forward in a circle to swish it round the internal surface of the tube, and finally finish swishing it in a figure-of-eight motion to get the ends.

Any excess WD40 will evaporate over time, so that's nothing to worry about; the only thing to worry about is that you should definitely do this outside, as the WD40 will drip everywhere. It's also worth giving the outside a quick wipe with a cloth to pick up the excess too - it'll make it less greasy to the touch as well.

Thanks for setting me straight, [MENTION=469965]Koyote[/MENTION] - much appreciated