Exterior Coatings for Glass

Q. We manufacture glass parts for exterior uses ranging from sunrooms to greenhouses. We have customers starting to inquire about adding various types of coatings for things such as anti-shatter films, additional UV protection and waterproofing, to name a few. However, other than hand-applied specialty films, we do not currently have a process in place to begin applying any type of coatings to glass. Would it be possible to recommend a finishing process for coating glass?

A. Coating glass is not difficult to do, but it does require several things be in place to insure successful results. Without knowing the specific type of coating(s) in question it would be difficult to give you exact specifics. However, I will try to highlight a few general approaches to what is needed in a glass coating operation. First, the main objective in glass painting process is to properly clean and dry the surface from any contaminants (even new glass). Surprisingly, simple common cleaners such as white vinegar are particularly effective in cleaning the surface. There are also a ton of commercial glass cleaners designed to remove grease, and other types of contaminants as well. With any cleaning process, it is very important to make sure the surface is dried sufficiently before applying any coating. Next, you will want to set aside a clean spray room environment to do your spraying. Ideally, you want the spray room area or shop to have a relative humidity of less than 50-60 percent. Too high humidity can definitely affect how well material will adhere to the surface. Next, you will want to spray in a room that is sealed off from the rest of your operation with proper filtration, which would trap any particles that may enter into the spray booth area. The last thing you would want is to have debris that could ultimately end up on the fresh wet coating.

I would also highly recommend that your spray pump station be located outside of the spray booth as well, with pass-through connections for only the air/fluid lines to the spray guns inside the spray booth area. This will additionally help keep the interior of the booth from collecting additional dirt and debris in the spray booth interior area.

The next component of a successful glass coating is the spray operation. You have probably heard the saying “You get what you pay for;” well, this is especially true when selecting a spray gun/pump system. This is one area where it typically never pays to be skimpy with the budget.  Many hand spray operations today use HVLP (high volume low pressure) spray guns ranging from gravity feed to air-assist airless technologies. Without knowing the exact properties of the coatings in question it would be difficult to recommend an exact spray gun technology. Your local sales or service representative can point you in the right direction in regards to proper spray gun technologies; tip sizes, pumps, etc., needed for your future operation.

It is equally important to understand that there are physical properties in paint that can affect how well it is sprayed or transferred to the surface in question. First, the temperature of the material is important. Temperature affects the viscosity of material, and if it is too cold it may not atomize correctly, resulting in a rough, undesirable textured application. It is always recommended to follow your material manufacturer’s specifications for proper spray temperatures, material reducing, etc. before applying their coating materials.

There are also a few basics you’ll need to understand in regards to the application of the material, regardless of the type of spray gun technology used. First, is not to spray too close to the surface. You more than likely will be spraying these parts lying flat, and if you are spraying too close to the surface, you’ll over deliver coating material that may run off the edge. Your coating supplier can recommend the proper mil thickness rate that the material needs to be applied at. A mil is one thousandths of an inch, and as a general rule, an average pass of a medium viscosity coating will range between 3-5 mils wet.

A good rule of thumb is to spray approx. 8-10 inches from the surface while keeping the spray pattern perpendicular (90 degrees) to the surface while you are spraying. You want to start the triggering of the spray gun a few inches before the surface edge and release the trigger a few inches after it leaves the edge. The spray pattern should overlap each pass by about half the width of the spray pattern, until the surface has been completely coated.  There are some instances you may need to apply a second coat over the previous wet coat at a right angle 90 degrees to insure proper mil thickness and uniformity to the coating manufacturer’s specifications.

Last, if you want to keep the overspray from adhering to the backside of the surface you are spraying, it would be a good idea to paper mask the back surface of the glass. Good luck.