I understand that I can apply a retainer to a project.

I work with a client that sends me various projects at a time and prepays most of the time. Is there a way to apply a retainer to a "client" instead of a "project" so that no matter which project I work on under this client, I can deduct it as I go along?

Thanks

I don't think so ... am in the same situation as you. Resolved it by making a "Retainers" project which holds all retainers categorised by name.

When working on the actual projects and if there is some additional invoicing to be done I use the "Apply retainer" functionality as you know it.