PPGI and PPGL coil supplier inspection in China
Colour-coated coil fails in ways that are hard to see in a photograph: colour drift between batches, thin paint, poor adhesion and a substrate that is not what was quoted. Here is what we check on the line.
Why colour-coated coil is a verification problem, not a colour problem
Buyers usually choose PPGI and PPGL on colour and price. The failures that cost money are almost never the colour you approved: they are colour drift between batches, paint that is thinner than specified, adhesion that fails after forming, or a substrate that was swapped without anyone saying so.
None of those are visible in a sample board photographed under a phone light. They are visible in the plant: which substrate goes into the line, how the paint is applied, whether the oven is running at temperature, and whether the factory keeps batch records at all.
The product, briefly
Pre-painted coil is a coated steel substrate with a primer and a top coat applied on a continuous coil coating line, then cured in an oven. PPGI uses a galvanized substrate; PPGL uses a zinc-aluminium substrate, commonly Galvalume. Export specifications usually sit between roughly 0.2 mm and 1.5 mm thickness, with RAL or custom colours.
| Parameter | Why it matters | Where it is confirmed |
|---|---|---|
| Substrate type | Galvanized or zinc-aluminium changes corrosion and heat performance | Mill certificate, line observation |
| Substrate coating mass | Hidden under paint, so it is rarely re-checked by buyers | Certificate, third-party test |
| Primer and top coat thickness | Drives durability and is the easiest thing to reduce quietly | Line records, laboratory test |
| Paint system | Polyester, silicon-modified polyester, PVDF and others behave very differently outdoors | Certificate, supplier declaration |
| Colour and gloss | Batch drift is the most common after-sales complaint | Batch records, retained samples |
| Back coat | Affects appearance of the reverse side and forming performance | Line setup, sample |
What we check on site
- Is there a coating line? Uncoiler, pre-treatment, primer coater, top coater, curing oven, cooling and recoiler. A trader with a sample room is not the same thing.
- Which substrate is loaded? Galvanized or zinc-aluminium, and whether the coil tags match what was ordered.
- Pre-treatment. Cleaning and conversion coating before primer, which is where adhesion problems start.
- Oven and line speed. Curing temperature and line speed together determine whether the paint actually cures; running faster than the oven allows is a classic quality shortcut.
- Colour control. Whether the factory retains batch samples, measures colour, and can show you the records for the batch you are buying.
- Packing. Coil wrapping, edge protection and the packing standard used for sea freight.
A question worth asking before you order
"If we reorder this colour in six months, how do you guarantee it matches?" A factory with a real colour control process will answer with retained samples and batch records. A trader will answer with reassurance. The difference matters on a project where panels are installed in stages.
Substitution patterns in colour-coated coil
- Thinner paint. A lower film thickness than quoted, which only shows up in a laboratory test or after a few years outdoors.
- Substrate change. PPGI supplied where PPGL was ordered, with no note on the documents.
- Primer omitted or reduced. Cheaper, faster, and it fails as adhesion rather than appearance.
- Colour drift accepted. The first coil matches the sample board, the third one does not.
All four are addressable before shipment: agree the specification in writing in both languages, verify the line, and book a third-party pre-shipment inspection. See sourcing support.
Verifying a supplier for this product?
Send the supplier name or city and the specification. You get a written scope and a fixed fee before we travel.
PPGI and PPGL questions
What is the difference between PPGI and PPGL?
PPGI is pre-painted galvanized iron, meaning a paint system applied over a galvanized substrate. PPGL is pre-painted galvanized steel with a zinc-aluminium substrate, usually Galvalume. The substrate changes corrosion behaviour and heat resistance; the paint system changes colour, gloss and adhesion. Both are quoted as pre-painted or colour-coated coil, so the substrate has to be stated separately in writing.
Why do two batches of the same RAL colour look different?
Paint batch, primer, line speed, oven temperature and substrate all affect the final colour. Colour drift between batches is one of the most common complaints in this product, and it is why we ask to see the coating line, the colour samples and the batch records rather than just a sample board.
Do you check paint adhesion?
We observe and record the factory's own adhesion checks and the surface preparation where they allow it. Formal adhesion and flexibility testing to a standard is a laboratory matter, which we can coordinate with a third-party inspection body.
Can you verify a coating line that only does toll coating?
Yes, and it is a common arrangement: a coater applies paint to substrate owned by a trader. In that case the identity question becomes more important, because the company selling to you may own neither the substrate nor the line. We report exactly what we saw.