An apparatus, including a flexible insertion tube having a distal end for insertion into a body cavity, and first and second conduits configured to deliver first and second fluids, respectively, to the distal end. The distal end includes a first balloon coupled to the first conduit so that the first fluid inflates the first balloon and is delivered, via one or more spray ports in the first balloon, to tissue in the body cavity, a second balloon contained within the first balloon and coupled to the second conduit so that the second fluid inflates the second balloon, and multiple splines including a flexible, resilient material and extending along a longitudinal axis of the distal end, and configured to constrain the second balloon so that inflation of the second balloon creates lobes that form, along the longitudinal axis between the lobes, channels that direct the first fluid to the spray ports.