Plainer rhymes with:
complainer, nonplanar, planar, planer, complainer, klejna, nonplanar, planar, planer, aigner, brainer, braner, breighner, campaigner, complainer, container, craner, cranor, daner, drainer, entertainer, feighner, gainer, gainor, gayner, gaynor, graner, hainer, haner, hayner, kainer, kaner, klejna, kraner, mainer, mainor, maner, mayner, maynor, nonplanar, planar, planer, rainer, rayner, raynor, retainer, saner, schaner, shaner, stayner, tainer, trainer, trainor, traynor, wainer, waner
Plainer sounds like:
palmer, palmeri, palmero, palmieri, palmira, palmore, palomar, planar, planer, planner, plenary, plenmeer, plumer, plumeri, plummer, pollner, polymer, pulmonary