Ray asks: What is the easiest way to mask railway wheels when spray painting them?
The simple answer for me is: Don't bother.
Because all the locos I build have black wheels, I don't bother masking them off. The chassis is painted once I'm happy with the running. Then it's dismantled, cleaned, primed and painted.
If I'm feeling clever, the wheels get a coat at this point too. A quick shot of primer and then the same paint as the chassis. Treads and flange backs are polished clean - this process can be made a lot easier if you remember to wipe some WD40 from the no-mess pen on the parts to stay shiny first.
I'm just as happy to paint the wheels by hand when they are back in the chassis. Cleaning treads is then carried out by flipping the loco upside down in a Peco cradle and running it while fibre penning the paint away. If it's sticking really well, I'll lightly apply a scalpel to the treat as it spins but very lightly - the paint wants to be removed, not any metal!
All of this would be different for a set of lined wheels - you'd have to do this before re-fitting them or go mad playing "dodge the waggly bits". I might also think twice if I was using Gibson or Sharman wheels. Slaters and especially Romfords can be treated in a caviler manner such as this with no ill-effect whereas plastic centred jobbies might complain a bit more.