I just binned one of these, I’ve had it for 5-6 years but rarely use it preferring a Herschel wedge, but hold it up to the light and check for pinholes. I stored mine indoors in a clean plastic bag in a foam case, when I had it new I noticed a cluster of pinholes but a couple of weekends ago I got it out and the result is the picture below. According to Seymour the coating is on the back, (I thought it was sandwiched between layers of glass) and they recommended using a black permanent marker pen to cover them, something the good people on the Solar Chat forum which the BAA is a sponsor recommended not to do as it could endanger your eyesight and the camera chip. Since I haven’t used it for quite sometime and it’s remained in its case indoors, I’ve no idea how the pin holes became so large, perhaps flaking over time around the edges? as they were nowhere near as large as this the last time I used it. The filter in the picture below fits a 90mm refractor, I drilled a very small hole in the dew cap which the screw passed through as it nearly blew off in a gust of wind.
I did ask Seymour if the coating degrades over time or if it was flaking around the edges caused by incorrect storage, ( can’t see how I could have stored it any better) but they only suggested the black marker pen. Out of curiosity when they told me the coating is on the back I tried to scratch it with my fingernail, but couldn’t make any impression on it.