The board should vote for an assessment to cover the cost of painting. An assessment for painting the exterior is based on the % of the association each unit owner owns. If your deed says you each own one-third of the association, you would each pay a third. You might set up a payment schedule that works for everyone.
In many if not most states, right before a condo is sold, the board signs off on a document (in Massachusetts, it's called a 6D) indicating if the unit owner owes any money. The new owner prior to the closing will review this document to see what s/he is getting into.
If a unit owner does not pay the assessment, then typically, a condo association files a lien on the unit making it difficult for owner to sell without first paying their debt. To file a lien, you would most likely work through a lawyer, but I have been told the cost of this is not much.