Quick Answer

Ships need a minimum of 300 microns of dry film thickness (DFT) of primer to adequately protect steel in corrosive marine conditions. This typically requires 5 or more coats by brush. Applying fewer coats is the most common painting mistake onboard — and the main reason repairs fail prematurely.

Why Thin Coats Fail

The primer coat creates a barrier between steel and moisture. When that barrier is too thin, moisture works through to the steel and corrosion returns — often within months of a fresh paint job.

The most common misunderstanding onboard: maintenance manuals typically specify 2 coats at 150 microns DFT each. This is misread as “2 coats by roller,” which delivers only 60–100 microns total. That is roughly one-third of the required protection.

The result: the repair looks complete, but the steel is under-protected. Corrosion reappears faster than it should.

How Different Tools Apply Paint

Application method matters. Not all coats are equal:

  • Brush: approximately 50 microns wet film thickness per coat, with better penetration into the steel surface profile
  • Roller: approximately 30 microns wet film thickness per coat — not suitable for the first coat, as it does not penetrate into the surface valleys

The first coat must always be applied by brush to ensure the primer reaches into the rough steel surface and bonds properly.

The 5-Coat Rule

To reliably reach 300 microns DFT using a brush:

Apply a minimum of 5 coats of primer. Each coat diagonal to the previous one. This technique fills the peaks and valleys left by the brush, building even coverage across the surface.

If in doubt — over-apply.

In 35+ years of maritime maintenance, significantly undercoated repairs have been seen thousands of times. Overcoated repairs that failed because of too much primer: approximately 2–3 cases.

The risk of under-application is much greater than the risk of over-application.

How to Check Coating Thickness

Two tools are commonly used:

  • Wet Film Thickness (WFT) gauge — used after each coat to check coverage as you go. Less reliable on pitted or uneven steel.
  • Dry Film Thickness (DFT) gauge — measures the total dried coating. Available in digital or magnetic (ex-proof) versions for use in tanks and confined spaces.

Checking DFT at multiple points after each coat prevents the most common mistake: discovering under-application only after the topcoat has been applied.

Frequently Asked Questions

What is DFT in ship painting?

DFT stands for Dry Film Thickness. It measures the thickness of a dried paint coating in microns. For primer on ships, a minimum of 300 microns DFT is the standard required to provide adequate protection against corrosion.

How many coats of primer does a ship need?

Typically 5 or more coats applied by brush to reach the required 300 microns DFT. The exact number depends on the paint product and application method.

Can you use a roller for primer on ships?

Not for the first coat. Rollers deliver approximately 30 microns per coat and do not penetrate the steel surface profile. A brush must be used for the first primer coat. Rollers can be used for later primer coats or topcoats.

What happens if you apply too little primer?

The coating barrier is too thin to prevent moisture from reaching the steel. Corrosion returns faster — often within months — undoing all the preparation work.

How do you check if you have applied enough paint on a ship?

Use a Wet Film Thickness gauge after each coat and add the readings. After drying, confirm total coverage with a DFT gauge. For brush application, applying 5 coats diagonally to each other reliably reaches 300+ microns.