Is it detectable by PSA?
No, if you use a gel formulated for grading, without residue, without odor, and if you wipe it correctly. PSA does not penalize "gentle cleaning".
Can I also clean the back?
❌ It is very risky. The back is often more fragile, and PSA is very attentive to it. Only do it in case of an emergency (visible stain), and with great caution.