I have a couple questions:

Why are you testing for lead?

Do you have children 6 y.o. or younger? or, are you pregnant?

I want to establish a risk assessment before attempting to answer this question.

If your houses legally tests negative for lead, that is very good for you for a number of reasons which I will explain later.

My wife and I are trying to have a child so hopefully soon on for both.

I'm looking to remove wallpaper and repaint. I know that will involve sanding. It already involved that in 2 of the bedrooms that we had done when we moved in. They most certainly didn't worry about lead and even cut into the drywall to remove mold near the windowsill.

But those rooms, as well as our 3rd bedroom, all tested -0.2 to 0.2. The calibration was -0.1 to 0.1 so I can see that the bedrooms are likely at zero lead and it makes me feel better about how painters we had handled the rooms and how I can handle doing the 3rd bedroom myself.

The bathroom, which also has wallpaper (and some glossy paint on wanescoting), also tested 0.2 and under so that seems safe to do as well.

Still do wonder about the appropriate prep precautions.

The one hallway wall that had those various results, one as high as 0.7, we are leaving alone for now. Just not worth dealing with and it's not ugly wallpaper. Still, I want to hang pictures and have drilled into it once before to secure furniture.

But given the range of results: 0, 0.2, 0.4 and 0.7... and the fact that all of our walls otherwise tested basically 0... I'm thinking it was just within range of error for the XRF?

I suppose I could cut into the wall to take / send a sample if concerned. Not sure why just one wall would have lead and nothing else?

The door jamb makes sense. It was an old exterior door that had an additional built on to it. The one wall? Makes less sense.

As one more point of reference. Our exterior paint tested 0.0 on our garage but 0.5 on the siding. It is definitely new paint and lead-free. So that shows me that some "higher" results can show up on lead-free surface.

The biggest concern is the staircase. I haven't yet found anyone I trust to do it right now before my wife is pregnant. But I'd rather someone do it right and have my wife (and myself) live with her parents during that time that they do it. Then appropriately clean everything that could be impacted (i.e. get professional to clean the carpets and ensure they HEPA vacuum not just the staircase enclosure but also the basement below (might be good time to just clean the entire basement floor and re-pour the concrete).

The other option is to either 1. do nothing or encapsulate myself without scraping. It isn't a high traffic staircase (being the attic) so even if the paint doesn't stick perfectly it's better than nothing. But really leaning towards getting this damn crappy lead-painted staircase out for good. Quotes I've had to tear down and rebuild were not much more than scrape/encapsulate professionally.

Sorry for the long rant!

It is good you are being cautious now, as children 6 and under are most at risk for lead poisoning. As you probably know, airborne dust is most problematic (next to lead fumes which is probably not going to be a factor), and therefore sanding when a young child is present is an example of things to avoid. Ingestion is the primary way lead enters the bloodstream. Knowing this, just keep the mess contained, and then clean up your mess.

For the sake of simplicity, I am going to state that adults are more or less immune to the effects of lead dust. Knowing this, doing all your work now is the right idea.

You can isolate the area (use zip walls if you need an easy way to do this) vacuum carefully afterwards, discarded all plastic sheeting, and wipe down surfaces with wet-naps when done, and you're golden. Relax.

*removing wallpaper should not involve sanding. If done correctly the wallpaper paste remover should do most of the work (happy to walk you through it if needed).

Note - if you send in a sample to be tested, you are legally required to disclose the results to any and all future buyers of your home.

With sanding I suppose I more meant that after wallpaper is off I'm likely to need to sand the paint below to ensure smooth and primer adheres well to it.

Gardz is a great sealer. It is specifically designed to seal wallpaper residue.

https://www.rustoleum.com/product-catalog/consumer-brands/zinsser/primer-sealers/gardz-problem-surface-sealer/

Afterwards, a skim coat could be applied to correct imperfections or create a smooth surface prior to painting.

*wallpaper removal: work one wall at a time. Keep DIF wet at all times with large roller. May take up to an hour before paper starts to fall off. Use putty knife and/or pull paper gently off wall. Careful not to gouge surface. Should pull off in large sheets, but sometimes important to remove paper 'face' first. Do not use paper tiger (it will damage wall and makes it harder to pull of paper later). Rinse with warm water and sponge when done to remove as much adhesive residue as possible and let dry. Seal with Gardz or similar.

good luck with your improvements!