Drywall – Can dried out joint compound be rehydrated

I don't have any experience with boxes, but have quite a bit with the 5-gallon buckets of compound. Without knowing much about how those boxes seal, I'd recommend going with a bucket- the buckets/lids seal air tight which is important for keeping the mud from drying out. The other thing I'd recommend- every time you close the box or bucket, put a piece of plastic sheeting over the top of the mud. This traps the moisture in against the mud, and prolongs its shelf life. The buckets come with a round piece of plastic sheeting on top of the mud- I just fold it in half (to keep any bits of mud stuck to the plastic from being exposed to air) then when I am done for the day, unfold it, and put it back on top of the mud in the bucket.

Paper tape you apply mud first then bed the paper tape with mud still wet.

1. Apply a thin layer of mud slightly wider than width of tape
2. Lay tape over seam
3. Scrape the paper tape with a taping knife to bed the tape. A thin layer of mud will cover the tape in the process.
4. Feather the edges of the mud with a wide knife (10" - 12")
5. wait until dry and scrape down any ridges or unevenness with the taping knife
6. Apply a wider layer of mud over top and smooth it out and feather the edges.
7. Preferably repeat steps 5 and 6.

Fiberglass tape you apply before any mud.

Edit: One more tip....

For small patches like that I usually only worry about feathering the outside edges and will feather it out that edge to the width of my largest knife (12" I think.) When I'm done with that edge and everything is dry I'll come back and fill in the entire inside area over the patch with mud and level it out to the height of the mud over the tape. It makes the repair less noticeable. Instead of having a large outline of a rectangle of raised mud you have a square that blends in better.