Originally Posted by
Crake
WD40 will break down or displace other lubricants. And left in small spaces it will start attracting dirt and become tacky, you shouldn't leave it in door hinges, or skateboard bearings, or on surfaces youll be applying silicone based products on. I dont have a clue if bike chain lubricants are affected. Bike lube is already very thin it may not matter at all.
I wouldn't leave WD40 on any surface I will be treating with something else (paint, lube, caulk, adhesives, etc). unless I was sure its safe.
Diluted Isopropanol will dissolve WD40 (other things will too, but IPA is probably the safest and most readily available thing to use)
Anytime lubes, especially WD-40 comes up, things get in to the weeds pretty quickly.
WD-40 works well for cleaning stuff because it's a very thin oil, that's why it breaks up grease-buildup. It has some lubricating properties, especially on rusty stuff, thanks to its' ability to 'creep' but, that 'thin' viscosity means it's no good for long-term lubrication or exposed surfaces.
I like it for 'degreasing' things that are going to be lubed again, like chains and cassettes (freewheels) since degreasers like acetone and isopropyl alcohol will often remove old lube farther in than new lube will reach.
As far as the original sawdust issue, i'd spray (WD-40), wipe off the buildup, and apply your choice of fresh chain lube. Then go ride your bike.
Concrete or plaster dust, i'd probably get more agressive, but a little bit of sawdust is pretty harmless.