Scuff The Surface?
Q. I have a metal surface that has to be electrocoated. Is it important to scuff the surface before paint is applied? J.U.
A. From your question, I assume you are a fabricator rather than a coater. One of the beauties of e-coat is that “if it is clean and conductive, it will coat.” This is a simplistic statement, but not far from the truth. To answer your specific question, no, the part’s surface does not need to be scuffed before paint is applied.
Depending what the e-coat is to provide to the part, the type of e-coat, type of pretreatment and any additional coatings will be determined. The most popular e-coat (used by most custom coaters) is black cathodic epoxy e-coat paint with zinc phosphate pretreatment. This combination provides excellent corrosion resistance for parts that are not subjected to direct sunlight. Being an epoxy resin base, sunlight will cause the coating to “chalk.”
To give you a better understanding of what e-coat can be used for, the following excerpt is from previous Clinics discussing “Design Criteria.” The idea is to determine the correct resin system and type/size of e-coat system you need for your parts.
A) To determine the correct resin system, ask:
What is the purpose of the coating?
- Corrosion protection
a) Epoxy if not exposed to direct sunlight.
b) Epoxy if used as a primer for liquid or powder topcoat. Two-coat e-coat is also available.
c) High-grade substrate and pretreatment with acrylic coating if exposed to direct sunlight.
- Color retention and selection
a) Significant volume (square feet of product surface to be coated) is required per color.
B) To determine the capacity of the material handling, pretreatment, coating and curing systems needed, ask:
What is the volume, size and weight (pounds and maximum part thickness) of my parts?
- Maximum part size (length × width × depth) will determine package size.
- Minimum part size may determine labor, tooling, and material handling, because e-coat can handle significant part density versus liquid or powder (if the part gets wet, it gets coated).
- Volume of parts (per color) will help determine system throughput (line speed on monorail systems, or carriers/hr on square transfer systems).
- If part size and volume varies significantly, labor, tooling and/or material handling will be affected because loading and unloading the line will vary. Power-and-free conveyor systems can help resolve this problem but are quite expensive.
- Maximum part weight and material thickness will probably determine the curing cycle (time and temperature) and the need for forced cooling. If the part weight and material thickness varies significantly, over-curing of lighter-weight parts can affect color and gloss (as will line stoppages).
This is probably more information than you need, but will hopefully give you a better understanding of what questions to ask when someone asks for “e-coat.”
This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 13, 2012.
Question: I am responding to the article in the January 2001 issue regarding the comparison between powder coat and electrocoat performance.
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