What is the recommended topcoat for a "pre-primed" metal door? Customers are sometimes surprised by the answer. The safe approach is to lightly sand and apply an acrylic metal primer (80227, 80228), followed by two coats of an acrylic enamel topcoat (17 series) semi-gloss.
"Pre-primed" or factory primed metal doors are usually highly crosslinked to generate a fast, hard cure allowing quick handling and shipping of the doors. While some are formulated with a zinc or other active rust inhibitive type system, many only offer a barrier type approach to preventing rust during shipping. These type systems can readily break down very quickly in the absence of a quality topcoat. Due to these variables it is beneficial to properly prime the substrate, after lightly sanding, promoting a good plane of adhesion. Some metal doors may contain an oily residue, which should be wiped away with acetone prior to sanding.
Periodically, manufacturers of "pre-primed" metal doors will develop problems associated with adhesion of topcoat system as well as the factory applied primer itself. The buyer of the door has no method of determining satisfactory results until the topcoat has been applied and has had sufficient time to totally cure. Often with age, the primer will lose adhesion to the door, resulting in total delamination of all the paint films. This is usually related back to the highly crosslinked system mentioned earlier.
If crawling occurs during application (new coat separating while being applied or edges moving in towards center), it is usually the result of the incompatibility of the new coating over the surface of the substrate. By proper cleaning, improved results can often be expected, but in some cases, only cleaning and priming with the correct primer is the key for success.