  Diffusion of elements in the atmosphere and envelope of a star can
drastically alter its surface composition, leading to extreme chemical
peculiarities. We consider the case of hot subdwarfs, where surface helium
abundances range from practically zero to almost 100 percent. Since hot
subdwarfs can form via a number of different evolution channels, a key question
concerns how the formation mechanism is connected to the present surface
chemistry. A sequence of extreme horizontal branch star models was generated by
producing post-common envelope stars from red giants. Evolution was computed
with MESA from envelope ejection up to core-helium ignition. Surface abundances
were calculated at the zero-age horizontal branch for models with and without
diffusion. A number of simulations also included radiative levitation. The goal
was to study surface chemistry during evolution from cool giant to hot subdwarf
and determine when the characteristic subdwarf surface is established. Only
stars leaving the giant branch close to core-helium ignition become
hydrogen-rich subdwarfs at the zero-age horizontal branch. Diffusion, including
radiative levitation, depletes the initial surface helium in all cases. All
subdwarf models rapidly become more depleted than observations allow. Surface
abundances of other elements follow observed trends in general, but not in
detail. Additional physics is required.
