FT Archives

Roobol On Painting

Friction-Reducing Paint Sought

Can you tell me the name of the fluoro-urethane friction-reducing paint supplier mentioned in your October column? I have an application I would like to try it on.

The coating is a fluoro-urethane, developed jointly by the Naval Research Labs and the Electric Power Research Institute. The coating is hydrophobic, has a coefficient of friction slightly less than Teflon, and has been in use for about 20 years. The cost of the coating-$350 a gallon-has kept it relegated to specialty markets. Another supplier has a similar product at www.astridmm.com/sea-slide/sea-slide.html that you should test also.

Painting Over Powder Coated Metal

How can I paint over metal that has been powder coated?

First, I will assume the surface to be coated is thoroughly clean. A number of powder coatings can be painted over directly, using more of the same powder coating, a new powder, a solventborne or even a waterborne paint.

In other instances (and this is more likely to happen) the adhesion will not be satisfactory if no additional preparatory steps are taken. When that is observed, one should try a light scuff sanding of the powder coating before repainting. In most cases, this surface abrasion will enable good intercoat adhesion. Should this remedy fail, it will be necessary to try another paint to use for recoating over the existing powder coat.

Estimating Software Programs

Is there such a thing as a quick estimating software program on the market that may ask for sizes of parts to be coated so it can figure the surface area, the type of material being coated, whether paint or powder will be used, the cost of the material being used and what we charge per square foot?

The Powder Coating Institute does have a form that approximates what you are asking, although it is not a software program. It's in Virginia and can be reached by calling 703-684-1770, or on the Web at www.powdercoating.org. One or more powder coating suppliers may also have a version of this form. Remember, the form is only a rough guide and some judgment needs to be used to apply its results to actual circumstances.

Nondestructive Thickness Testing on Plastic

We need to paint roughly 450 complex parts made of an ABS plastic blend with a urethane enamel. We have tested adhesion, and it is excellent. The only suitable way to paint this limited number of pieces is by manual spray application. It is not a big order and we can easily paint it all in less than three days. My problem is that for part-performance reasons, our customer needs us to apply the urethane at 1.5 to 2.0 mils; any more or less is not acceptable. They are willing to pay us well to achieve the type of accurate film thickness they have specified, and for us to test each part 100% in four specific areas. I need to find a nondestructive method to determine dry film thickness.

To check film thickness on plastics, we normally just attach a small square of thin sheet metal and use a magnetic pull-off gauge. Then we repair the spot unless it is a scrap part. But our customer will not permit any repairs. Is there some test method you know of that we can use but that doesn't damage the paint?

Two methods for nondestructive film-thickness testing on plastic substrates are available: beta-ray back-scatter (electron bounce-off) and ultrasonic wave reflection. Neither one is absolutely guaranteed to be able to differentiate between the paint and the plastic beneath it, but if they can differentiate, the test will provide extremely accurate film-thickness measurements. Beta-ray backscatter uses an instrument costing around $6,000 to $10,000. The sonic tester is priced at about half that much.

Since you only need the test device for this job of fewer than 500 pieces, you might wish to explore with instrument companies a short-term rental of one of these devices rather than an outright purchase. I doubt that in the future you would ever receive many jobs for which the film thickness is that critical, so buying a tester would probably not make economic sense.