Small Color Sets and Warped Shadow Masks
 

As I understand it, various older small color CRTs have a problem with the shadow mask warping and thus displaying a subtle wavy pattern on screen.

Is there any way to correct this issue?

?? Reference ??

There are two kinds of shadow mask warping, neither of which produces a "wavy' pattern, but instead a color purity problem.
1) thermal warp from high beam current - can occur with a constant area of high brightness, but reverses when the beam current is lowered
2) mechanical warping from dropping the set - permanent

By wavy pattern, do you mean moire'? This is a design issue of dot size vs. scan line spacing, and is permanent for a particular tube design.

Yes, here's a vid I took showing the subtle pattern around the screen. Be sure to view with good resolution, as it's subtle and hard to capture on cam.

https://youtube.com/shorts/6nk-TYi0JOM?feature=share

Image example:
https://ibb.co/k6b1kw0

I've seen that on a 23" deltagun before but not on any of my portables. Not sure what it's called either. Best guess is it's some kind of moire pattern caused by minor misalignment between shadow mask and phosphor and not a warp in the shadow mask.

It's the moire' pattern between the very coarse dot pattern on that tube and the scanning lines. It goes away if you defocus the tube, but that just makes the picture bad in a different way.

By the way, making a video of it can introduce more moire' from the interference between the dot pattern and the video camera pixels; and then the video coding can mess it up some more.

You have to tell us if the YouTube video looks like the picture on the set or is worse due to these added effects.

Indeed, the dot pitch is coarse. Good suggestion-will experiment with the focus and see if I can subtly tame the pattern.

The best representation of the problem is the still image posted in my last comment.