Is it 'normal' polish or Shellac?
If Shellac, I do nails first - then straight on to the tan. No Problem
If 'normal' polish, I do all the nail prep and filing first, then get them to undress and apply tan. (They risk smudging the polish when they undress if the polish is applied before tanning. )
I then carefully wipe the nail plate and apply polish after they have been tanned and are dressed. I ask them just to sit at the edge of the chair so as the tan doesn't wear off the backs of their legs.
This works for me.