Put on long-sleeved clothing.
Spread a drop cloth around the tub.
Scrape away the silicone caulk from around the borders of the tub with a razor utility knife, making sure not to scratch the tiles.
Put on latex gloves and a ventilator. Clean the inside and outside of the bathtub with muriatic acid to release and remove oils from the bathtub surface. Scrub the acid with a bristled cleaning brush.
Tape a plastic drop cloth over the tiles with tape to protect them from the bathtub paint.
Keep your latex gloves on. Spread an acid etching paste over the tub using a sponge or soft towel. The paste helps the primer bond better. Allow the paste to set for 15 minutes and then wash it down the tub.
Fill an air gun paint sprayer with the primer made for use on a bathtub. Spray the entire tub with the primer and allow it to dry. Once the primer has dried, apply the top coat in a rustic color of your choice using the air gun paint sprayer.