Pinholes in Polymer Ceramic Material
We manufacture electrical insulators and other electrical equipment from a polymer ceramic material consisting of 87% silica in various grain sizes for compaction, and a methyl methacrylate monomer. The material is not fired, but cures (20 minutes) at room temperature through a chemical reaction. During the chemical reaction, tiny pinholes appear in the surface of the material. They are so small it is hard to see them, but when we coat the material with a fluorourethane coating, the pinholes become very visible. We have tried to fill them with a paste material before painting them, but this procedure is less than successful. It does not fill the holes as the paste material either does not enter the tiny pinholes and the air pressure in the holes just pushes the material back out. When we apply the fluorourethane coating by spray gun, the pin holes appear to be even larger than they were originally. Even the second coat has pinholes in it.
Is there some undercoating that can be applied to the polymer ceramic material that will cover the pinholes, allowing a smooth topcoat?
Undercoating will not work. Vacuum impregnation will fill the holes with an inert material. That's one way to seal porosity.
But you have not told me how you polymerize the acrylate ester. You merely said it "cures." If you are able to reveal more on this step I could possibly show you how to avoid pinhole formation. It might also be possible that the fluorinated urethane coating application can be altered to seal the pores.
Restoring Old Signage
I greatly enjoy reading your Roobol on Painting Q&A column in Industrial Paint and Powder magazine. I have learned a great deal about the best practices in applying and maintaining industrial paints and coatings.
I have a particular problem with some "classic" bus stop signs that I thought you might be able to help me solve. The signs are 30-plus years old, of which 10 of those years they were exposed to the seasonal variations of a Montreal climate. The signs are in good condition with the exception that one of the paint colors (purple) used in the company's logo is showing signs of oxidation and fading. The questions that I have therefore are as follows:
- What are the best ways to reduce or stop the continued oxidation of the paint on the sign?
- Are there any coatings that I should consider using to protect the paint from further degradation?
- Are there any waxes or polishes that I should consider using to restore the oxidized paint to a shiny "near new" condition?
- The signs feature a glossy, three-color company logo (red, blue and purple) that is painted on a glossy white background (no decals or reflective materials were used for either the logo or the background).
- The sign appears to be made of stainless steel.
Thanks for the kind comments on this magazine. As to the signs, my best advice is not to do anything just yet. I doubt you will even want to make these look "near new." Certainly before you do anything you must talk with a dealer or museum curator knowledgeable on their value. Changes you imagine would enhance their appearance might actually destroy much of their worth to collectors. If the signs are determined to be valuable and would truly benefit from being touched up, an art restoration expert should do the work.
Reader Response to E-coating Screen Mesh
You had a question on how to avoid filling openings with paint when E-coating screen mesh. I have been in the E-coat business for a long time. The person trying to coat the 180-mesh screen will have much better luck using lower voltage. The coating does not see a screen as a solid mass but the coating will "throw" across the screen openings at higher voltages.
Many thanks for your response. I missed that idea in my original answer. Reduced voltage certainly should be worth trying, although this will reduce transfer efficiency.