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Paint strainers

Posted: Saturday, September 10, 2011 11:55 AM Quote
Here is some really good information I found online about paint strainers

Passing any type of paint through a mesh strainer is part of effective particle control. Paint jobs are ruined when they contain hunks, bits and chunks of hardened paint or other matter.

Traditional paper cone paint strainers come in several mesh sizes. A “medium” mesh strainer has filter media woven roughly 44 by 36 threads per inch, or 225 to 226 microns. A “fine” mesh paper cone filter has media woven 60 by 48 threads, or 190 to 200 microns. “Extra fine” strainers, most often called for with waterborne coatings, have holes that are 125 microns. One micron is equal to one millionth of a meter. A 200-micron hole is larger than a 125-micron hole.

Ordinary paper cone strainers are constructed of mesh from material that won’t shed fibers. Less expensive strainers use cotton mesh, and more expensive strainers use some type of synthetic material like nylon. The heavier the paperboard stock of the cone, the more durable the strainer.

The thickness of paper is stated in pounds – the weight of 500 sheets of paper cut into a specific size, often 24-inch by 36-inch, so 500 sheets of 80-lb. stock would weigh 80 lbs. Inexpensive strainers use thinner paper stock and could collapse into the mixing container under the weight of the paint being strained. When nearly all auto paints were solvent-based resins, the glue that held the cone together could be made from various components. Now that many auto paints contain water, the glue must be waterproof.
Disposable Cup Systems

Many painters have adopted a disposable cup system that has a built-in paint straining filter. These speed-friendly products have been around for nine years. I found six different cup/strainer systems made by four different manufacturers that are commonly sold for collision refinishing. In general, their purpose is to save painters labor time in mixing and applying the paint and cleaning the equipment. Available in a variety of container sizes from 3 ounces to 34 ounces, some containers are measured in portions of a liter.

A disposable cup system is comprised of the container for the paint, a lid to keep it inside, sometimes a collapsing bag that makes clean-up faster, possibly a quick coupler that  joins the cup to the gun, and a strainer to clean the material to be sprayed. Some systems employ a cylindrical “barrel” filter that snaps into the throat of the fluid inlet, and others use a flat, round filter screen that has much more surface area.
Waterborne Chemistry

The issue with waterborne chemistry and micron size pertains to how water resins cure. They dry by coalescing together to form a film, unlike solvent resins which dry by evaporation of the solvent into the air. A key difference is that a clotted hunk of solvent resin will re-dissolve in the presence of more solvent. A dried particle of waterborne resin that has clotted into a hunk won’t re-dissolve. As a result, several paint brands call for the smallest hole size (125 microns) to ensure that the final paint finish is perfectly smooth.

Remember that the change in paint chemistry is driven by the environmental rules enacted to clean the air we breathe. The amended Rule 1151 from the South Coast Air Quality Management District (SCAQMD), which covers most body shops in southern California and has become the model for many other regulatory bodies, doesn’t call for latex paint resin. It calls for basecoats that contain less than 3.5 lbs. of volatile organic compounds (VOC). Several paint brands have perfectly compliant coatings that still contain some solvent, along with or instead of water. If the basecoat still has solvent, dried chunks will be more likely to re-dissolve and can be suitably filtered with a 200-micron hole in the mesh.
Goal: Remove Particles

No matter which size sieve the paint manufacturer calls for, the goal is to remove any particles that can ruin the finish. As always, the biggest risk of particle contamination isn’t from the mixed color but from the dirt/dust on the vehicle/painter. A tack rag unfolded to its full size and wadded into a loose ball is the best way to ensure the surface is clean; a disposable paint suit will keep the dust on the painter from falling onto the car.

The mesh screens in disposable cup systems (often made from fiberglass) can also plug up faster because of the viscous nature of waterborne resins. Thicker paint takes longer to pass through any filter and is more likely to plug the holes shut, hence the change from barrel filters to disk filters in several brands. More surface area means less plugging.

As painters using waterborne paints  have discovered, dried color is hard to remove from the metal parts of the gun and, as the water resins dry, they quickly choke the filter screen closed. In a way, it’s reassuring that those resins, when properly applied to the vehicle, will form a super durable coating!

Writer Mark Clark is a well-known industry speaker and consultant. He’s celebrating his 22nd year as a contributing editor to BSB.
Posted: Monday, November 21, 2011 8:09 AM Quote

How about a follow up to your Do you recommend a specific brand or micron for your customers?
Posted: Monday, November 21, 2011 3:18 PM Quote
Both PPG and Gershon make 125 micron filters and you should be able to buy them locally at the auto paint supply store or online. Paint filters are cheap so buy lots and avoid re-using them.