Question: I'm painting all my walls so I started prepping the surfaces by sanding. When I got to a peaked joint I proceeded to sand it with an electric orbital sander and paint started peeling more and more until it got to this state:

[![Large patch with peeling paint 1](https://i.stack.imgur.com/zgIwh.jpg)](https://i.stack.imgur.com/zgIwh.jpg)
[![Large patch with peeling paint 2](https://i.stack.imgur.com/VylVC.jpg)](https://i.stack.imgur.com/VylVC.jpg)

I googled around and found this question: [How to prepare wall with peeling paint for painting](https://diy.stackexchange.com/questions/85339/how-to-prepare-wall-with-peeling-paint-for-painting)

From the answer posted, I tried hand sanding the peeling edges but the paint layer is removed at a slower rate than the white material underneath, resulting in the surface dipping at the edge:

[![enter image description here](https://i.stack.imgur.com/ZpLCF.jpg)](https://i.stack.imgur.com/ZpLCF.jpg)

Should I proceed to sand with my orbital sander until the paint stops peeling instead? And what would be the best way to fill in this patch?

Answer: 
Sister another joist and add some more blocking. Second joist will cut down the width to 16" or just over, and blocking will help prevent additional flex that Trex has.
We just used [Fiberon composite decking](https://www.homedepot.com/p/Fiberon-ArmorGuard-15-16-in-x-5-1-4-in-x-8-ft-Nantucket-Gray-Square-Edge-Capped-Composite-Decking-Board-BRDVC-NG-8/202594357) to build deck stairs and the deck itself. As my first stairs project, we only put 3 stringers with about 20 inches on center.

The deck itself had 16 in OC span of joists. The difference is noticeable but not significant (IMHO). It seems that 17in OC is negligible difference especially since you will fasten rather long spans of the decking.

Adding blocking (screwing board into the joist) is definitely a solution. If you have access under the deck you can do it after the installation, if you find bounce problematic.