If they just sell a job for us I give them 10% but I run the show when it comes to our work since they aren't usually insured for our kind of work. I just tell them to have the customer pay us directly and we cut them a check when we get paid.
If they insist on a sub arrangement, I make sure we get paid when we are done, NOT when they are done. If you are a general contractor, you don't pay all your subs when the project is done, you pay them as you go and get a draw from the customer as the project continues. If your painter doesn't like it tell them to work it into their bid deposit so they can cover your portion of the power washing or have them simply add a notation on the contract that says the customer will pay $XXX when the power washing is completed.
If they insist on paying you at the end of the project, then go down to your clerk of courts and ask them to run the painters name and see if anything comes up on a public record search. Any time we are considering a sub job from another contractor we run them first to make sure they don't have any pending suits or judgments against them from customers or other contractors. We also run a D&B if it's over $1,000. After that make sure you get a contract signed by them.