WFT vs DFT: hitting the spec while the film is wet
Dry film thickness is what the specification accepts, but it can only be corrected after the fact. Wet film thickness is the number the applicator can act on in real time with a notched comb gauge.
The two are linked by one ratio: WFT = DFT / volume solids fraction. A coating that is 70 percent solids by volume loses 30 percent of its film as solvent evaporates, so hitting 5 mils dry requires about 7.1 mils wet.
The arithmetic
Volume solids is on the product data sheet. Divide the target DFT by the solids fraction to get the WFT target: 4 mils DFT at 65 percent solids needs 6.2 mils wet; 10 mils DFT at 80 percent solids needs 12.5 mils wet.
Thinning changes the ratio. Adding 10 percent thinner by volume dilutes the solids, so the WFT target rises accordingly. If thinning is permitted at all, recompute solids as original solids divided by (1 + thinner fraction).
Choosing the target: spec midpoint
Aiming at the specified minimum leaves no room for normal application scatter - half the film ends up thin. Aiming at the maximum risks runs, sags, and solvent entrapment. A practical default is the midpoint of the specified range, which is what CoatSpec's WFT helper offers.
Read the WFT immediately after application in a few representative places. Solvent starts leaving the film at once, so a delayed comb reading understates what was applied.
Frequently asked questions
How do I convert WFT to DFT?
DFT = WFT x volume solids fraction. A 8 mil wet film of a 75 percent solids coating dries to about 6 mils. In reverse, WFT = DFT / solids fraction.
Why aim for the middle of the DFT specification?
Application always scatters around the target. Aiming at the midpoint keeps normal scatter inside the specified range in both directions, instead of pushing half of it below the minimum or above the maximum.
Does thinning change the WFT target?
Yes. Thinner dilutes volume solids, so the wet film needed for the same dry film increases. Recompute the solids fraction for the thinned mix before setting the comb gauge target.