Photoetched brass is cleaned to stop the etching process and most are then packaged without too much additional handling, or those doing the handling take typical precautions such as cotton gloves to prevent skin oils from being transferred
If you open your packet with the PE, handle it by the edges and paint it right away, you can probably get away without cleaning it.
If you have pawed the PE with your pizza grease stained fingers you may need to give the PE a soak in lacquer thinner. Allow to air dry. I have done this, but normally have not found it necessary.
Some may also suggest a bath in a mild acid, such as vinegar, for several hours. Rinse in water & air dry. This will slightly etch the brass & give it some tooth to hold the paint. I have never found it necessary to do this.
Don't sand the PE. This may possibly deform the parts.
Toms PE is etched from a soft brass. It will be more liable to be deformed by an errant thumb than PE by other manufacturers (White Ensign, Gold Medal Models, Flagship, or Eduard).