FT Archives

Roobol on Painting

Coloring These Walls OK

We are students who want to paint a mural on a wall in the library of our school. We are wondering if it is OK to paint over the old paint.

To be sure the mural paint will adhere well to the existing wall paint, scrub the surface thoroughly with detergent and then carefully rinse it off with clean water. Then you should test the mural paint adhesion by applying some onto a small area. Let it dry for 24 hours, then try to peel it off by affixing Scotch tape to the paint and quickly pulling it off.

Uneven Film Thickness at Edges

I'm developing an automated dip-priming process for small, alodined-aluminum detail parts that have geometries suitable for dip application. I'm using a 2K chemical-resistant epoxy that is formulated with acetone as its primary solvent-the automatic viscosity control holds the viscosity constant using acetone. On paper, the one-gallon paint tank is small enough to be economical when compared to manual spray painting. I'm trying various combinations of air-knife blow-off, agitation rates, single vs. double dipping, viscosity settings and withdrawal rates to achieve the best possible finish of 0.3 to 0.8 mils dry film thickness.

My problem is that I'm seeing a narrow band of surface adjacent to the upper edges of the parts with very little or no primer, far less than on the flat surfaces or edges. It looks like gravity and perhaps the air-knife blow-off (for blowing out holes and reducing drip-bead on the bottom edge) are causing the wet film to slide down the surface slightly, leaving the bare band.

Can you suggest other possible causes of this phenomena or potential fixes? I wanted you to know that I've purchased your Industrial Painting book, and it is one of the first resources I go to when I am troubleshooting a paint problem or investigating new applications.

The problem is hard to evaluate without seeing where the air knife is positioned. In the absence of an air knife, the defect is due solely to paint rheology, which in turn is affected by both the surface tension and the viscosity of the paint. You are seeing why conventional dip-coat processes (as opposed to E-coat or chemiphoretic coating) are rarely used to paint parts with high appearance quality.

I have a suggestion that may help, however. Devise a way to turn the parts as they drain after being dipped so the paint film thickness is more uniform. This will reduce the fatty edge bead on the bottom as well.

I'm pleased you find the second edition of my book helpful to your work. But more detailed and newer information can be found in the edition that was just released. You may wish to call 800-950-8977 to purchase Industrial Painting & Powdercoating, Principles and Practices, 3rd Edition.

Flowcoating Difficulties

We refurbish used electrical transformers, and this includes repainting the housings that contain louvers for internal transformer heat dissipation. We grit blast off all the old paints down to bare metal and then flowcoat the housings in an outdoor space with no booth or hood for solvent containment. We use an oil-based paint. Our problem is that the paint dries up too rapidly and doesn't provide uniform coverage, so we are constantly adding solvents to adjust viscosity. The dry paint film cannot be too thick because it would then reduce the opening size of the louver slots and interfere with the housing's air ventilation. Can you suggest a way to correct our difficulties?

Since you coat a maximum of three to four housings per day, I cannot imagine why you are trying to flowcoat them. And I believe your paint is likely to be an air-dry solventborne material, not an oil-based resin system. You said when I called that the plant has three air compressors.

It is probably not legal to use flowcoat without solvent vapor exhaust, even in your rural location. Set up a simple exhausted paint booth and a pressure-regulated, compressed-air supply with good water, oil and particulate filters. You can then air spray the housings using a conventional or HVLP cup gun. This will give superb film thickness control and enable you to avoid excess paint on the louvered areas of the housings. The minimal amount of paint used may allow you to paint without the need for regulatory permits, but check this out ahead of time so your company does not incur environmental protection violation fines.

More on Friction-Reducing Coatings

I was forwarded a question about friction-reducing coatings asked by a naval architect in your column. Apparently these coatings are not familiar to these end users or to you. My company holds patents for such hydrophilic coatings, which are routinely used in the medical markets for ease of insertion and leaching of antimicrobials and drugs. In fact, we also supply the recreational boating market with a temporary "go fast" coating that is hydrophilic. We wondered if you would like some commentary on these coatings? Of particular interest were the thoughts put forward by the NPCA group that met in Virginia Beach with a large contingent of Navy and Coast Guard personnel about marine and offshore coatings. Please advise if we can help in this matter.

Thank you very much for your response with information on this topic. Many readers would certainly be interested in learning more about such coatings and hearing comments concerning their usage. I'm eager to find out the specifics of their performance.