I find that painting the inside of the blank tends to work better. The texture of the drilled hole seems to hold the paint better and as mentioned, any bubbles are between the paint and the tube, not between the blank and the paint. If you are impatient, and the paint is still soft, it is possible to scrape it off when putting in the tube, and the ca seems to facilitate this to a degree by softening the paint. Either let the paint get really dry (like overnight) or another thing I found is that you can chamfer the outside of the end of the tubes just enough to break that corner, and reduce the tendency. I use a reloading tool intended to deburr and chamfer cartridge casing mouths for reloading. That don't fix it 100percent of the time, but it helps.