The Protective Coatings Glossary (SSPC 00 – 07) defines a “Stripe Coat” as: “A coat of paint applied only to edges or to welds on steel structures before or after a full coat is applied. The stripe coat is intended to give those areas sufficient film build to resist corrosion.”
SSPC Paint Guide No. 11 defines “Stripe Coat” or “Striping” as: “A way of providing extra corrosion protection measures on edges, outside corners, crevices, bolt heads, welds, and other irregular surfaces, including optional surface preparation techniques for sharp edges to improve coating performance.”
Surfaces that are Typically Stripe Coated
Typical surfaces that are stripe coated include:
- Back-to-back members
- Nut and bolt assemblies and rivets
- Inside/outside corners
- Corrosion pitted steel
- Built-up structural members
- Other irregularly shaped surfaces
The above surfaces are typically stripe coated because they contain “edges” and “crevices” to which a uniform coat of paint is difficult to apply. When applied to edges, surface tension and shrinkage of the coating during drying tends to draw the coating away from the edge resulting in a thinner applied film. Thinner coating films mean reduced corrosion protection and an increased susceptibility to damage. Crevices are typically a combination of “outside corners (edges)” with a crevice in between. While the same surface tension and shrinkage stresses apply along the edges, surface tension also inhibits “flow-out” of the coating down into the crevice.
Application of Stripe Coats
Brush application is typically the preferred method for applying stripe coats but rollers and spray application are sometimes employed. Brush application helps the coating overcome surface tension and forces the coating downward into crevices rather than bridging over them. Airless or conventional spray are sometimes used to deposit the wet material on the surface, which is then immediately followed by “back brushing” to work the coating in. When properly done, spray application with back brushing provides better uniformity and thickness of the stripe coat. Conventional spray is sometimes preferred over airless spray for stripe coating because the fluid and air flow adjustments on equipment gives the applicator more control over the application process.
Cost Verses Effort – Is it Warranted?
Not always. Structures in mild exposure environments or interior exposure environments may not need the added corrosion protection of a stripe and therefore, the extra cost and effort of stripe coating may not be warranted. However, the cost and extra effort is typically worthwhile when the structure: is in immersion service (water or chemicals); is in highly corrosive environments; has a history of pitting corrosion, crevice corrosion, pack rust formation, and rust bleed problems; and/or includes complex structural configurations such as built-up box or lattice box-type members, complex gusset connections, rivets, nut/bolt assemblies, etc.
Structural designs/shapes that Minimize the need for Stripe Coats and/or Improve the Effectiveness of Stripe Coats
There are some structural design considerations that can help minimize the need for stripe coating and improve the effectiveness of stripe coating when applied. Those considerations include; edge grinding (to minimize surface tension), using full seal welds versus open-type stitch welds (to minimize crevices), designing structures using tubular members rather than I-beams (eliminating edges, inside/outside corners) and strategically placing weep holes to facilitate water drainage and prevent ponding water on the structure. Unfortunately, all the above improvements can be costly and labor-intensive, which often precludes their use.
Stripe Coat Application – Before or after Full Coats
When applying stripe coats to blast cleaned steel surfaces the stripe coat is more commonly applied after the full primer coat has been applied. Attempting to apply the stripe coat to blast cleaned steel may result in rust back of the steel surface before the full coat can be applied. However, applying the stripe coat prior to any subsequent full coat typically results in improved appearance of the overall coating system versus applying the stripe coat after the full coat. Applying the stripe coat before the full coat makes it easier to visualize the surfaces being stripe coated, improves localized protection, and is easier to inspect. Applying the stripe coat after full coats typically necessitates the use of contrasting colors to make the stripe coat visible to applicators and inspectors.
Stripe Coat Application – Wet-on-Dry Verses Wet-on-Wet
Industry experience has shown that both ways are used. Wet-on-dry stripe coats must typically dry for a certain time-period before the full coat is applied and can involve any coat. The dry times typically conform to the manufacturer’s “Dry-to-Recoat” interval listed on the product data sheet. Wet-on-dry stripe coating is typically a separate and distinct operation that requires more time to apply, but allows better visibility for inspection. Wet-on-wet stripe coating typically requires special approval from the manufacturer because the full coat is applied as soon as the stripe coat has released most of its solvent and is still “tacky” to the touch. While wet-on-wet stripe coating is less time-consuming it requires that the inspector be present during application if the installation of the stripe coat is to be visually verified.
Both methods achieve the intended goal, which is the application of extra thickness and insurance of complete coverage on the surfaces being stripe coated.
Thickness of Stripe Coats
Stripe coats are typically not measured because the irregularly shaped surfaces to which they are applied cannot be accurately measured with dry film thickness gages. The main goal when applying stripe coats is to control the thickness to be as thin as possible while still achieving complete coverage of the surface. Applying the stripe coat too thick can result in film defects such pinholes and cracking, and may also result in aesthetics issues.