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DA Marks

Ever really thought about sanding? The sight of this on a yacht might make you…

This image shows “DA” marks adjacent to the hull rub rail sponson on a large yacht. (DA is short for Dual Action. A machine sander that both oscillates and rotates)

The marks are the direct result of poor technique. On a new build project there can be little discussion regarding apportioning responsibility for these.

On a refit project the applicator will always default to the argument that these marks were present before the refit so they are not responsible. Unless there are some very good pre-refit image records then this is a hard argument to win.

If such marks are identified prior to repainting then efforts can made to correct them. Skilled pre-refit paint surveys carried out by someone with an expert eye for such details will benefit all parties.

Regardless to any of the above, however, we simply don’t want to be employing applicators that will put such marks into the side of a yacht. It is entirely avoidable and unnecessary. The DA machine operator just needs better training and attention to detail.

What is also really sad about studying this image is that I can almost picture the person carefully moving the machine along by the cove so as not to sand into it, yet that “care” has lead them to inadvertently weighting the edge of the sanding pad and creating these semi circular marks.

Understanding “shimmer”

The images below are deliberately taken at the very worst possible lighting and viewing angles in order to highlight the topic discussed. Images 1 and 2 of “Yacht 1” are taken from the showcoat stage of a 70m+ superyacht build .

In image 1 the reflections of the scaffold in the paint finish to the right show a highly rippled effect on the paint surface. This is commonly referred to as “shimmer” and nobody wants to see this on their yacht.

The prime cause is poor use of “DA” or Dual Action random orbital sanders.

A DA sander has a flat circular pad onto which abrasive sanding discs are mounted.

The sanding pad’s central spindle moves eccentrically at a user adjustable speed of between 4000-10000 orbits per minute depending on machine. The pad is also free to rotate (i.e Dual Action) which results in a randomly orbital sanding scratch pattern.

A DA cannot be used entirely flat without it shaking. This is because the operator’s hand must always apply a force to counter the rotational force of the machine. This results in a slightly higher pressure to the right side of the pad so as to counter the clockwise rotation. Hence the vertical shimmer in Image 1.

In image 2, where the surface is convex, the pressure on the sides of the pad is relieved by the curvature of the surface and so the top or bottom of the pad will now exert the greater pressure and we see horizontal shimmer lines instead of the vertical ones in image 1.

While DA sanders offer significant advantages in terms of efficiency and speed compared to manual sanding methods, it is crucial to ensure that operators are properly trained and understand the importance of applying even pressure and maintaining consistent movement patterns. This is particularly critical in the context of finish sanding, where the goal is to achieve a very even, flat surface.

Sanding a surface as evenly as possible using a DA requires moving the sander quickly and evenly across a wide area of the paint surface in a manner similar to using a sanding board.

Many operators fail to do this and merely sand slowly from side to side across a 20-30cm wide zone rather than more rapidly across an 80-100cm wide area.

Image 3, of Yacht 2, shows another yacht that has been DA sanded multiple times prior to the application of topcoat but here the level of shimmer seen is minimal. These sander operators knew their craft.

In conclusion, while DA sanders can contribute to a shimmer effect on paint surfaces, the real issue lies in the training and expertise of the operators. By addressing this, and other techniques, through a comprehensive training and certification program, the superyacht coatings industry could raise the standard of quality and craftsmanship across the board.