Accuracy degradation is difficult to quantify because it depends on how much and how evenly you spray.
The attached document is a study done by the University of Michigan, that evaluates the effects of powder spray on optical measurements. They used a White light system.
The bottom line is that they used a laser profilometer to measure the topography of the sprayed surfaced and found that a sparsely sprayed surface can have high spots of up to 100microns (possibly caused by large droplets) and that a more dense evenly sprayed surface could have a highs of up to 62microns. Low spots were measured in the 40 to 50micron range.
As long as you apply one nice/even coat, 50 microns (0.002") should be a good estimated average coat thickness.
Click here to download: Evaluation of Powder Spray Effect
llp, scanarm, laser line probe, spray, coat, accuracy