I am pretty sure he has our sliders. My apologies that this is happening, or if you do not have our sliders anymore. Then it is someone elses problem.

Anything can rust and powder coat is not the magic solution. Some areas as worse than others. DC (and a lot of the east coast) is probably one of those.
That said, we had some issues with the early design StepSliders that showed up over time. In the tight areas where the top plate meets the tube, you would not get coverage due to the powder not sticking in the areas due to the charge from both sides repelling the coating when applied. Also, the textures powder coat might be a little more porous than the regular semi black.
That is the main reason we have changed the sliders to have a bolt on top plate. We did not anticipate this happening, but it did. We can only improve it from the feedback we get. Ideally we should have done a 5 year salt wash test to make sure that there is no corrosion, but that is simply not possible.
I would suggest you clean the areas out with a wire brush to get rid of the rust, then treat with a rust converter and top coat with some Rustoleum. Use a small can and small brush.
We do our best to have the coatings done as best we can with the best prep, but sometimes you can not prevent it from happening. We tried to zinc platings before you do powder coat in the past, but this created other issues with acid (from the prep) getting trapped in the tubes and leaking out of small pin holes. This would happen when the parts were put into the ovens during the powder coat process. When this happens, the acid eats the powder coat and create a bigger mess.
The only true way to do it is to do galvanizing before coating, but then you end up with a pretty rough texture. It was also cost prohibitive to do the two processes.
What does the rest of the truck look like underneath?