Finish Adhesion Failure

There are two places where a finish can fail to adhere. The finish can fail to stick to the substrate (substrate adhesion) or it can fail to adhere to the previous coat in the finishing system (inter-coat adhesion). There are two types of adhesion that affect a finishes ability to stick to a surface: mechanical adhesion and chemical adhesion. Mechanical adhesion to the substrate is affected by how you prepare the surface, sanding or planing. A surface can be too finely sanded or too smooth for some finishes. Inter-coat adhesion is affected by scuff sanding between coats or the ability of some finishes to burn in to themselves or chemically bond. Both types can be affected by contamination.

In my experience most adhesion problems occur with the modern finishes, polyurethanes, waterbornes or two part finishes. Adhesion failures with shellac, lacquers or traditional varnishes seem very rare. Problems when mixing waterborne and solvent borne products in the same schedule are common.

Here is a partial list of causes for adhesion failure in no particular order.

  • Excessive substrate moisture content
  • Improper sanding procedures- polish sanding substrate
  • Incompatible coatings within the finishing system
  • Insufficient curing and dry times.
  • Contamination of substrate
  • Extreme temperatures
  • Excessive pigment load in stains
  • Omitting scuff sanding between coats with certain finishes
  • Natural oils and resins in teak, pine, etc.                                                                                                                                                                                                                                             Below are two pictures of a desk, it was over coated with a polyurethane varnish in an attempt to spruce it up. With out adequate mechanical adhesion, I was able to remove the entire top coat with a single edge razor blade.

This is a rocking chair that someone stained with a heavy coat of oil based stain and then top coated with a water borne varnish, then I got to work on it.Same rocker. It was stripped and refinished with lacquer based products.

About millcrek

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