Why does paint lift at a sharp edge?

The first place paint fails is almost always an edge. The cause is not the paint — it is the geometry of the edge.

Six months after a block is painted and sent to site, where does the first rust show? Not in the middle of a plate. At the cut edge, at a flange mouth, at the corner of a stiffener.

This may not be the coating contractor's fault. Usually it isn't.

Paint pulls back from an edge

Wet paint is a liquid, and like any liquid it has surface tension. On a flat surface the film thickness stays roughly even. At a sharp edge the behaviour changes: the liquid draws back over the convex corner. Once cured, the film at the edge may be less than a third of the thickness measured on the face.

So a system you measured at 320 microns may be sitting at 60–70 microns on the edge. Without cathodic protection, corrosion starts there and runs along the edge while the rest of the surface is still sound.

There is a second problem: a sharp edge is also mechanically exposed. When the block is moved, when a sling passes over it, when a welder steps on it, that is the first place to take damage.

What the standard says

ISO 8501-3 defines preparation grades P1, P2 and P3 for edges and weld seams. P1 is the loosest, P3 the strictest. Marine and offshore projects usually call for P2 or P3, which means sharp edges must be removed.

Coating specifications are more explicit. NORSOK M-501, widely used for offshore coating systems, requires sharp edges to be rounded, typically to a minimum radius of 2 mm. Follow whichever specification applies to your project — but where a figure is given, it is usually around R2.

The problem with dressing by grinder

The most common method on site is to knock the edge back with a grinding wheel. It works, but it has three problems.

• The radius varies with the operator. One end of a block reads R3, the other has almost nothing.
• You cannot measure it. When the inspector arrives you have no figure to show, and the argument starts.
• It is slow and tiring. On a long edge, quality drops as the operator tires.

The wheel itself is a consumable too; per square metre it is not as cheap as it looks.

Carbide-insert edge rounding machines

Edge rounding machines cut rather than abrade. The head carries indexable carbide inserts and removes material to a fixed radius from the edge. A guide roller follows the plate edge, so all the operator has to do is feed the machine along.

The difference this makes: fit an R3 head and the edge comes out R3 — at the start of the block and at the end. You have a figure to show the inspector.

Because the inserts are indexable, a dull cutting edge means turning the insert rather than replacing the head. On a four-cornered insert that is four cutting edges from one insert.

Before you start

Choose the radius from the coating specification. If the specification is silent, ask the client's coating contractor. If you are torn between R2 and R3, take the larger one — it will not cause a problem on the coating side.

One more thing: laser and plasma cutting leave a hardened layer at the edge. A radius head takes that layer off without trouble, but insert life will be shorter than on an oxy-cut edge. Plan your insert stock accordingly.