When I painted the underside of another car, I wire brushed and treated any rust or flaking stonechip, then wiped all underneath with degreaser to make sure clean, gave it a good coat of zinc primer with a brush, seam sealed with brush on sealer and used a aerosol stone chip where required, then to finish it off I had some synthetic enamel mixed to car colour and applied this all underneath with a small gloss foam roller, due to ripples in stonechip it looked as good as spraying over it and has lasted very well. Only really worth spraying if doing concourse in my opinion. Beam and arms etc are better being blasted and powder coated.