Mill finish, PE film, coated backing or fully coated — four states, four different jobs. This note says which one belongs on which face and what to write on the order.
Mill finish with a PE protective film on the face that will be laminated is what most sign blanks need. The film is not decoration: it keeps the rolling surface free of scratches and handling marks until the reflective sheeting goes on, and it is peeled immediately before lamination. Ask for coated backing only when the panel will sit in Gulf-level sun or on a coastal structure, and for double-sided film only when both faces are visible during fabrication and transport.
There are only four states a blank leaves a factory in: bare mill finish, mill finish with PE film, coated on the back, and fully coated. Everything else a buyer is offered — "chromated", "degreased", "protective coated" — is a description of one of those four. The choice follows what happens to the panel next, not what sounds most protective on a quote sheet.
Mill finish is the surface as it comes off the rolling mill: a thin, natural oxide layer, no applied coating, a slight rolling texture. It is the substrate that reflective sheeting and printing inks are designed to bond to. PE film is a low-tack polyethylene sheet applied on the levelling line; it protects the surface mechanically and carries no chemical function. Coated backing means a powder or liquid coating on the reverse face only. Fully coated means both faces, which is a different product — a painted panel, not a blank waiting for film.
| Finish | What it does | Where it is the right call | What it does not do |
|---|---|---|---|
| Mill finish | Bare rolled surface, natural oxide | Panels coated or laminated within days, dry inland storage | No protection against handling marks |
| PE film, one face | Mechanical protection until lamination | Any blank that will be laminated after delivery | Does not stop edge corrosion or white rust |
| PE film, both faces | Protects both sides through fabrication | Double-faced signs, panels cut and drilled after coating | Adds a peel step and a waste stream |
| Coated backing | Reflects heat, seals the reverse face | Gulf, coastal and high-UV sites; dark posts | Not a substitute for edge sealing of the film |
One state per order line. If a quotation lists two finishes for the same panel, ask which face each applies to — that is usually where a specification drifts.
Reflective sheeting bonds to aluminium through a pressure-sensitive adhesive. That bond needs a clean, dry, oxide-stable surface with a defined surface energy. PE film keeps the surface in that condition; leaving it on during lamination traps air and dust, and any adhesive residue left by a film that has been in the sun for months will show as a haze under the sheeting. The working rule on our line is: peel, wipe, laminate — within the same shift.
This is also why film grade and film type get confused in specifications. "Protective film" in a bill of quantities can mean the PE carrier or the reflective sheeting. They are different products with different jobs, and only one of them is retroreflective.
When the reverse face will be exposed to sun or salt. A bare aluminium reverse absorbs heat, expands, and drives a temperature gradient through the panel; over years that gradient works on the adhesive line and on the film edge. A coated reverse evens the temperature and keeps the alloy away from chloride. Gulf specifications commonly pair a 3.0 mm blank with a powder-coated back for exactly this reason, and it is the same logic on a coastal gantry in any market.
What it does not fix is water entering at a cut edge. That is an edge-sealing question, not a coating question, and it is dealt with in the film and fabrication stage.
Write three things: the face to be filmed, the reverse finish, and the storage condition expected before lamination. A blank specified as "mill finish, PE film on the face, bare reverse, laminated within 30 days" removes almost every finish argument that arrives after delivery.
The surface preparation expected before a pressure-sensitive retroreflective sheeting is applied — clean, dry, free of oil and contamination — is set out in the application guidance of ASTM D4956, "Standard Specification for Retroreflective Sheeting for Traffic Control", published by ASTM International. Durability classes for the finished sign are set by EN 12899-1 from CEN.
Sources: ASTM International, West Conshohocken, PA · CEN, Brussels. Cited 30 October 2026.
No. It protects the blank through cutting, drilling and transport, then comes off immediately before lamination. Film left on a panel in the sun for months can transfer adhesive and leave a haze under the sheeting.
No — it means no applied coating. Mill finish is the correct substrate for reflective sheeting and for printing, and most regulatory sign blanks ship that way with a PE carrier on the face.
Yes, but say so explicitly: a fully coated panel cannot be laminated without abrading the coating back, so it is normally specified for panels that will be printed or left bare, not for reflective signs.
Not for plain sheet. Coated or painted sheet can fall under a different subheading from plain sheet, so keep the finish description consistent between the commercial invoice and the declaration.
Specification questions are answered by the same person who quotes the job.