Sheet gets marked on site constantly – a size, a location, a job number – and the standard approach, a felt-tip marker, does not work. The ink sits on the surface and comes off on the first thing that touches it.
Why pens fail on PP
Surface energy again. PP is non-polar and most marker inks are formulated for paper and card, where they soak in slightly. On PP there is nothing to soak into and nothing for the ink to key to, so it forms a film that wipes off.
Solvent-based paint markers are the exception and they do work – the solvent partially attacks the surface and the pigment is left in a film that has some grip. They smell, they take a minute to dry, and they are the right tool.
What a paint marker does and does not give you
It gives a mark that survives handling, weather and a wash cycle reasonably well. It does not give a crisp edge or a consistent line weight, and it fades under UV over a season or two.
For identification purposes – which is most site marking – that is entirely adequate.
Stencils, if you need repeatability
If the same mark goes on many sheets, cut a stencil from an offcut of the same sheet. It is the right material: thin enough to lie flat, stiff enough to hold its shape, and it does not react with the paint.
Cardboard stencils work for a few uses and then go soft from the solvent. Sheet offcuts last indefinitely, which is a good use for the trim from cutting corrugated plastic sheet.
Spray paint, and the one caution
Spray gives an even coat and it is fast for large marks. The caution is that some solvent-based sprays attack PP enough to craze the surface, which weakens the sheet locally. Test on an offcut before committing, and if the surface goes cloudy, use a different paint.
Alternatives that last longer
A printed label on a treated surface, or a thermal-transfer label, both outlast a hand mark by a wide margin and both are cheap in quantity. If the mark carries information anyone needs to read in six months, use a label rather than a marker.
For permanent identification, engraving or a hot brand works, and it is what we would suggest for anything that has to survive years outdoors.
Removing marks later
Paint marker comes off with solvent, and the solvent that works best is also the one most likely to mark the sheet. Test first. A mark that will not come off cleanly is usually better painted over than scrubbed.
Making a stencil that lasts
Cut it from a sheet offcut rather than card, and cut it with a sharp blade so the edges are clean – a ragged edge gives a ragged mark, and solvent wicks into the ragged edge and lifts it.
Keep stencils flat. One that has curled will not lie against the sheet and the paint bleeds underneath, which is the most common reason a stencilled mark comes out blurred.
What we can do
We supply PP corrugated sheet and can supply it pre-printed or pre-labelled if the marking is repetitive, which is usually cheaper than doing it on site. Tell us what you are writing on it and how often – we will tell you whether marking it yourself is the sensible answer.