Right, need to sand down my forks, and then wash/degrease them, so inevitably some dust/water will get inside. Is that OK? OK to just wash the insides out, and then leave to dry?
WD40 is a solvent-based water repellant. Whilst it won't do much damage as long as you thoroughly clean it off, solvent can dessicate rubber seals and will breakdown any grease over time. Use a teflon-based lube like gt85 or mucoff's 'bike spray'.