What Actually Happens to Compound in Extreme Heat
Joint compound cures through two processes: evaporation removes water, and the binders in the compound set into a hard film. In normal conditions (65-75°F, 40-60% humidity), those processes happen slowly enough that the compound cures from the surface inward in a controlled way.
In Las Vegas summer conditions, surface evaporation happens dramatically faster than curing. The outer layer dries and hardens while the interior remains wet. When the interior eventually dries and shrinks, it pulls against the rigid surface, creating cracks. In severe cases the seams can look fine at the end of the day and be split open by morning.
The problem is worse at seams because the compound there is thicker. Thin skim coats over the field are sometimes okay. A 1/4" thick tape coat at a butt joint in those conditions is almost guaranteed to crack without climate control.
What Darnell Told Me to Do
Darnell's advice was direct. Either work in the early morning before 9 AM with the garage door open and a swamp cooler blowing across the work, or run a window AC unit and keep the space below 80°F. He preferred the AC because swamp coolers, while effective in Las Vegas's dry climate, add humidity to the air and change the drying dynamics in a different way.
He also told me to mix compound thinner than usual, not soupy but definitely wetter than what I was used to. Thinner compound is more forgiving in dry conditions because it holds more water to work with before the surface skins over. Some guys he knew added a small amount of joint compound retarder to slow the process. He hadn't done it himself but said it worked for people who knew what they were doing with the ratio.
He also said to keep a damp sponge handy and if I noticed surface skinning starting, I could lightly mist the area with a spray bottle before it hardened completely. Not wet, just damp enough to buy another 20 minutes of working time.
Fixing the Cracked Seams
The six seams I had done that morning were in different states of failure. Two were fine. One had light surface crazing that I could sand out without re-taping. Three had actual splits that had opened up enough that I had to pull the tape and start over.
Pulling dried tape out of cracked seams is not fun. The compound underneath crumbled in places. I ended up using setting compound (hot mud) to fill the worst voids and let that cure fully before re-taping with regular all-purpose compound. That added a full day to the project.
Total extra cost for my mistake: about $60 in wasted compound and extra materials, plus two additional days of work. The window AC unit I bought and installed in the garage cost $220. I've used it on every project since. It's the tool I didn't know I needed.
Las Vegas Timing for Drywall Finishing
Darnell gave me a rough calendar for when you could get away without climate control, based on his years working in the valley. October through April, morning work with ventilation is usually manageable. May and September are marginal and depend on the specific day. June, July, and August are non-negotiable: you need AC running in any enclosed space where you're applying compound.
Early mornings and evenings help but the ambient temperature in the valley stays high enough that an un-air-conditioned space never fully cools down in summer. At 11 PM in late July, it's still 95°F outside in Henderson. The house might be 100°F if it hasn't been cooled.
This is a bigger issue for people buying older homes or doing gut renovations where the HVAC isn't running yet. New construction generally has temp power and contractors can get portable units. For a DIYer working on a house that's between owners or between systems, it's worth renting a portable AC before starting any finish work in the summer months.