4.3 Coating Properties and Calculations

Quick Answer: The CIP exam will test at least one WFT calculation. WFT = DFT ÷ volume solids (as a decimal). Volume solids is the percentage of the wet film that remains as dry film after solvent evaporates. You also need to distinguish pot life (usable time after mixing) from induction time (wait time after mixing before you can apply), and recognize amine blush before it causes intercoat adhesion failure.

Volume Solids and the WFT/DFT Calculation

Volume solids is the percentage of the wet coating, by volume, that remains as non-volatile solids in the dry film. If a coating is 60% volume solids, then 100 gallons of wet coating yields 60 gallons of dry film.

The fundamental formula:

WFT = DFT ÷ volume solids (as a decimal)

Where WFT is wet film thickness (mils), DFT is required dry film thickness (mils, from the specification), and volume solids comes from the product data sheet as a decimal (60% = 0.60).

Worked example: The specification requires 4.0 mils DFT of an epoxy intermediate. The PDS lists volume solids at 60%.

WFT = 4.0 mils ÷ 0.60 = 6.67 mils WFT

The applicator measures this immediately after application using a WFT comb gauge per ASTM D4414. If the applicator applies 6.67 mils WFT at 60% volume solids, the resulting DFT is 4.0 mils.

Second example: Specification requires 8 mils DFT of a high-solids epoxy with 85% volume solids.

WFT = 8 mils ÷ 0.85 = 9.4 mils WFT

High-solids coatings shrink much less — 9.4 mils WFT produces 8 mils DFT (only 1.4 mils shrinkage), versus the 60%-solids example (2.67 mils shrinkage). High-solids coatings are more forgiving of WFT error because the wet-to-dry gap is smaller.

If thinner is added, volume solids drops and WFT must increase.

Exam trap: Always convert volume solids from percent to decimal before dividing. 60% becomes 0.60, not 60. A common wrong answer is 4.0 ÷ 60 = 0.067 — off by a factor of 100.

VOC Regulations

VOC (volatile organic compound) content is the amount of solvent that evaporates into the atmosphere, regulated by EPA and state agencies. VOC limits are expressed in grams per liter (g/L) of coating, less water and exempt compounds.

The inspector must verify that the coating's VOC, as listed on the PDS, does not exceed the project specification limit AND the applicable regulation for the location and service category (e.g., industrial maintenance coatings are often limited to 340 g/L under EPA AIM rules; local rules can be stricter). A coating exceeding the regulatory VOC limit cannot be used. High-solids and waterborne coatings are the two main strategies for meeting tighter limits.

Pot Life vs Induction Time

These two terms are often confused on the exam but are distinct:

A coating can have both. Example: a two-component epoxy may require 30 minutes of induction after mixing, then have a 4-hour pot life at 75°F. The applicator mixes, waits 30 minutes, then has 3.5 hours of working time remaining. Both are temperature-dependent — warmer shortens, cooler extends. Never thin to extend pot life — thinner does not reset the reaction; the coating is still over-cured.

Recoat Windows and Intercoat Adhesion

Recoat window is the time interval during which a subsequent coat can be applied without special surface preparation. If the window expires, the previous coat may need roughening (sweep blast or solvent wipe, per the PDS). Missing the window causes intercoat adhesion failure — the new coat peels off the old. Recoat windows are temperature-dependent.

Intercoat adhesion is the bond between successive coats. Failure causes include amine blush, contamination (dirt, oil, salts), exceeding the recoat window, or applying a non-compatible topcoat.

Two-Component Mixing

For epoxy and polyurethane coatings, correct mixing is essential:

1. Mix Part A (resin) alone first to ensure uniformity.
2. Add all of Part B (hardener) to Part A. Never split a kit; off-ratio mixing produces soft, undercured, or brittle films.
3. Power-mix with a low-speed drill and non-air-entraining paddle, scraping sides and bottom.
4. Observe induction time if specified, then apply within pot life.

The inspector should verify the mix ratio on the PDS and check the applicator mixes full kits or uses a calibrated plural-component pump.

Amine Blush and Carbamation

Amine blush is a waxy surface film that forms on some amine-cured epoxies cured in cool, humid conditions. The amine reacts with carbon dioxide and moisture at the surface to form an amine carbamate (carbamation). The surface feels slick. Intercoat adhesion fails if the next coat is applied over amine blush — the new coat peels off in sheets.

Detection: wipe with a cloth dampened with MEK or xylene; waxy residue indicates blush. Remove by washing with warm water and detergent, then rinsing. Verify removal before applying the next coat.

Mist Coat Technique

A mist coat is a thin, reduced first pass applied before the full coat. Common uses: inorganic zinc silicate cure assistance (helps the silicate binder react on smooth substrates) and adhesion promotion on dense or glossy surfaces (wets out the substrate, gives the main coat a chemical anchor). Mist coat is typically a 1:1 reduction applied as a very thin pass, allowed to flash, then the full coat is applied.