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May 11 - May 17, 2015

When and why would I need to stripe coat if I am using an edge-retention primer?

Selected Answers

From Carlos Augusto Correia of PETROBRAS on May 20, 2015:
For a reliable paint scheme, it is advisable to grind all edges with at least one grinder pass. For a two-coat, anticorrosive paint scheme,  two stripe coats are usually specified. Even using an edge-retention paint, the stripe coat can not be waived, if we want to guarantee a reliable paint scheme. The difference in this case is that only one stripe coat will be applied  with the edge retention paint, between the first and the second full coatings. In this scheme, the first coat can be a usual anticorrosive paint,but the second must be the edge- retention type. In brief, the stripe coat is always necessary and should be applied by brush. What will vary is the number of stripe coats, depending on the anticorrosive paint type. This approach has been used in cargo and ballast tanks of oil carriers with success.

From Warren Brand of Chicago Corrosion Group on May 18, 2015:
I have not seen the data supporting the efficacy of edge-retentive coatings. Stripe coating is usually very fast and a conservative manner in which to ensure a proper coating application. Anyone have any hard data on edge-retentive coatings?

From MICHAEL DEATON of Premier Services on May 18, 2015:
If you look at any job that is breaking down, notice WHERE it's breaking down (normally all of the areas that should have been striped). In plain English,  if you want a paint job to last any length of time, stripe coat before you spray every coat....period!

From Michael Beitzel of Modjeski & Masters inc on May 15, 2015:
Stripe coating should be employed particularly whenever spray painting a structure. In addition to providing adequate coating thickness to edges, the other primary benefit of stripe coating is to work paint into seams between components or discontinuities or crevices in welds. For this reason striping of seams, welds and fasteners should always precede application of the first full coat of paint as seams and crevices tend to be bridged or missed during spray application. Striping should also always be applied by brush to work coating into seams and to mechanically remove any residual dust from surface preparation efforts, which typically clings or is trapped in these seams after blow-down operations and could cause poor bonding to the surface if coating is just spray applied. Rollers can be used for striping edges and can be applied before or after the first full coat, and this step is necessary even if edge- retentive coatings are applied. They will have improved edge covering, but they will always have lower thickness and afford less protection than the adjacent flat surface. Striping also helps insure that areas with difficult access for spray application are properly coated. Some will argue that striping prior to application of the full first coat will jeopardize the quality of the surface preparation and want to "save the blast" by spraying first. What happens then is these difficult-to-access areas are left uncoated and will rust back until they are striped some time after the full coat has dried. This is likely the reason these areas are typically the first areas to break down in the future, even though they are the most critical in protecting the structural connections from fastener or weld deterioration and the development of pack rust between member components.

From Adan Cabeltes of Mega Paint Corp. on May 14, 2015:
It is the human factor. Whether the coating retains on edges or not, we still have to make sure we do it right.

From Alfredo Claussen of Instituto Mexicano del Petroleo on May 12, 2015:
First of all, why do edges need to be treated differently?  When a coating undergoes polymerization, a contraction is experienced, and this results in a certain level of residual stress to be developed inside the coating, which, coupled with the stress concentration at edges, facilitates the initiation of microscopic cracks. One of the initial contributions to stress concentrations that result in crack development is  a reduced coating thickness at edges, resulting from the wet coat trying to retract because of surface tension in the still liquid coating. But polymerization then literally "pulls" the coating as it "dries," setting in the high stress concentration that gives origin to minute tears and cracks. Other places where cracks develop are crevices, non- ground welds and interior angles in structural profiles. The best way to avoid those cracks is to get a large radius, at least 1/8" or 1/4" preferably, but this is expensive or cumbersome. Therefore, the proper application of a stripe coat is the most practical and almost mandatory if one wants the system to endure a long time. What paint and coating manufacturers call "edge-retention" is based on formulations and additives that tend to reduce wet pull and reduce chemical shrinking, but stripe coating is more effective in real life, if properly done. There is special equipment available, designed and sold for easy application of the proper width of stripe coats, avoiding the usual material waste that results from trying to employ conventional equipment to apply the relatively narrow stripe. This equipment  includes either roller or brush applicators fed from small and very practical cartridges that use static mixer nozzles that guarantee the proper mixing ratios. Look for a technical paper that I wrote in 2010 about residual stresses in coatings and ways to solve this problem at, under the title:"Fenomeno de Generacion de esfuerzos residuales en recubrimientos..." I know it is in Spanish, but you can see the photos of the special equipment and some diagrams that you will find self-explanatory

From Trinidad Diaz of Keppel Prince Engineering on May 5, 2015:
Stripe coating is still important to gain proper DFTs in those hard- to-reach areas.

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