In this article, I will introduce what PPGI is, how the paint system is specified, where pre-painted coil is used, and how it differs from bare galvanized coil and from PPGL.
What is PPGI?
PPGI means pre-painted galvanized iron: a zinc-coated steel strip with a paint system applied on a coil-coating line before the coil is sold. The metal under the paint is galvanized steel. The paint is not a color sample stuck on afterwards. Roofing, cladding, appliance skins and light enclosures use it so the factory does not run its own paint line.
The coating under the paint is still a galvanized specification. A Z designation such as Z275, explained in the galvanized guide as 275 grams per square metre of zinc counting both sides under EN 10346, can sit under the paint. The drawing has to say so. Paint does not replace a missing zinc code, and a zinc code does not describe the paint.
How a pre-painted coil is specified
A coil-coating line applies a pretreatment, a primer and a top coat, and a back coat on the reverse. The top coat is a resin system. Polyester, often called PE, is the common general-purpose coat. Silicon-modified polyester, SMP, is used where the specification wants more weather resistance than a standard polyester. PVDF, polyvinylidene fluoride, is the system specified for long outdoor exposure when the drawing names it. These are not shades of one paint. A bend, a chalk rating and a color match can all fail if the resin is wrong, even when the color name matches.
Color needs a reference, such as a RAL number or the project standard, plus gloss. Dry-film thickness belongs on the enquiry when the drawing states it. Do not invent a micron value to fill the blank. The back coat is either a full service coat or a thin primer, and the two are not the same on a roof that can be seen from below. Sheet is the cut-length delivery. Coil is the delivery for roll forming. Thickness, width, coil weight and the steel grade still apply, because the paint sits on a steel that has to form.
What a stated paint system protects
The resin and the film, not the color name, decide whether the coil survives forming and weather.
A color that still matches after forming
Roll forming bends the paint. A system with the flexibility the profile needs keeps the color at the bend. A hard film that looked right on a flat sample can crack there.
Weather resistance the project asked for
Polyester, SMP and PVDF are specified for different outdoor duties. Using a general polyester where the specification says PVDF is a different product, not a cheaper version of the same coil.
A substrate the profile can carry
Thickness and grade carry the roof or the panel. The paint does not. A beautiful coat on a grade that will not form the profile is still the wrong coil.
When PPGI is the right order
Order PPGI for roofing and cladding profiles, sandwich-panel faces, appliance wrappers and enclosures when the paint is applied before forming. Coil is the normal delivery. Sheet is the delivery when the buyer wants cut lengths and will not unroll a coil.
Order bare galvanized coil when the factory paints locally or when no paint is required. Order PPGL, pre-painted aluminium-zinc, only when the drawing names an aluminium-zinc substrate rather than zinc. The PPGI series on this site is the zinc-substrate product. Do not rename a galvalume coil as PPGI to make the line match.
PPGI versus galvanized coil
Galvanized coil ends at the zinc, the spangle and the chemical treatment. PPGI adds primer, top coat and back coat. The comparison is whether the paint is already part of the purchased product. A buyer who will choose the painter, the color and the film later does not need PPGI and should not pay for a coat that will be covered or stripped.
Two PPGI coils of the same color still differ by resin, gloss, film thickness, zinc mass and back coat. A quote that says only blue PPGI cannot be compared with a second quote. The lines above are the comparison.
FAQ
These questions are the ones that turn a color request into an order.
Is a RAL number enough?
No. It identifies a color target. The order still needs the resin, the gloss, the zinc coating, the thickness and the size. Two resins can aim at the same RAL and weather differently.
What is the difference between PE, SMP and PVDF?
They are three resin systems. Polyester is the general coat. SMP is specified for a step up in weather resistance. PVDF is specified when the drawing asks for that fluorocarbon system. Use the one the specification names.
Is PPGL the same as PPGI?
No. PPGI is paint on galvanized, zinc-coated steel. PPGL is paint on an aluminium-zinc substrate. The metal under the paint is different.
Can the back coat be ignored?
Not when the reverse is visible or has to resist the same building environment. Say whether the back coat is a service coat or a thin primer.
Who confirms the film thickness?
The drawing or the project specification. If it states a dry-film thickness, copy it. If it does not, do not fill in a micron number. Confirm it on the enquiry.
What to send with the enquiry
State coil or sheet, the steel grade and thickness, the width, the coil weight if it is coil, the zinc designation, the top-coat resin, the color reference, the gloss, the back coat, and the film thickness if the drawing gives it. The PPGI series lists sheet and coil. The paint system and the size are confirmed on the enquiry before the coil is coated.