What Went Wrong
Phoenix humidity in July runs around 15-20% at midday. Joint compound is designed for something closer to 40-50%. The math doesn't work in your favor.
My first coat went on reasonably well, but drying was uneven. The edges set up before the center, creating little ridges I had to sand down. The second coat was worse. I couldn't feather the edges fast enough; they were drying while I was still working the middle.
By the third coat, I was fighting the material instead of applying it. The compound got chunky on the knife, wouldn't spread smoothly, and left a texture like stucco instead of a clean surface. I ended up sanding most of it off and starting over.
The Compound Graveyard
I went through three buckets of joint compound trying to get one small patch right. The first bucket sat open too long while I figured out what I was doing wrong, and the whole thing skinned over. The second batch I thinned too much trying to buy time, and it ran down the wall. The third time I finally got it right, but only after some advice from a guy at the Mesa hardware store who'd clearly seen this before.
What Actually Works
The hardware store guy, Phil, gave me a list of Phoenix-specific tricks that salvaged the project.
First, work early. Really early. I started at 6 AM before the house had time to heat up. The compound behaved normally in the morning cool.
Second, thin the compound slightly more than you would in a normal climate. Not runny, but looser than bucket consistency. This buys you working time.
Third, work in small sections. Instead of taping a whole seam at once, do two or three feet, smooth it, and move on. By the time you finish a section, the previous one is ready for the next coat.
Fourth, and this was the game-changer, put a humidifier in the room. A cheap evaporative humidifier from Walmart raised the room humidity from 20% to maybe 40%, and suddenly the compound acted like compound instead of quick-setting cement.
The Final Result
Once I adjusted my technique, the repair came out fine. Not perfect, but acceptable. The texture match to the orange peel on my walls wasn't great, but that's a separate issue from the heat problems.
Total cost for what should have been a $40 repair: about $120 in compound (three buckets, only one of which was usable), $35 for the humidifier, and probably $15 in extra sandpaper. Call it $170 in materials for a patch that would've cost maybe $150 to have done professionally.
When to Just Hire It Out
If I had to do significant drywall work in Phoenix during summer again, I'd hire a contractor. The pros here know the climate and have tricks I don't. They work fast enough that drying time isn't a factor, and they've got commercial humidifiers and other equipment that makes the job practical.
For small repairs, the early morning approach works. But anything bigger than a couple square feet in July or August? Save yourself the frustration.