If I have back acne, should I use face wash on my back?

Yes. Yes, you can treat acne on your back with any of the over-the-counter benzoyl peroxide and/or salicylic acne wash. Be ware that some people are allergic to benzoyl peroxide, so if you experience any itchy rash, see your doctor.
That is ok. Acne is acne. Whatever works on the face will most likely work on the back. Face skin is thinner and as a rule more delicate.

Related Questions

Can ponds face wash clear acne/pimples?

Maybe. Face washes can help clear acne pimples if they have ingredients such as salicylic acid, benzoyl peroxide, etc., but they can also dry out your skin. I am not familiar with the ingredients of ponds, but I know it used to be used as a cold cream to remove makeup. This would probably be too heavy for skin with acne and would not clear pimples.

I have back acne and on my face, how will selsun help and do I put some on, let it dry then wash for face? For the back we can just simply wash withit

NO. You will need specific acne medication and topical or oral antibiotics. See you physician for an examination and evaluation.
Back acne. You would do better by using a Benzyl peroxide wash on your face and back. It is better to use a 2.5 or 5 % wash if you can find them. They are over the counter now. This can take 4 to 10 weeks to work. If you don't see improvement see a dermatologist. We have many more products available to us. You don't have to live with the acne.