Apologies - missed out a couple of important bits:
1 Try to get good quality colour photos of the loco/stock you are weathering - this helps
2 Tap off surplus powder into a large box ( I use an O gauge coach kit lid) - that way you can re-use powder - if you think Yorkies are tight with dosh then Durham lads after 25 years in Yorks can really show how to be tight(!)
And please - try to avoid bright colours - it is rare to see these in real life - most of the locos I can remember (Q6, J27, WD) were greasy and never cleaned by West Hartlepool Shed - but didn't have anything bright in colour on them apart from wheel treads)
Hope this helps