Why is my glass mat bubbling after I wet it out?

The rapid cure time you're dealing with is almost certainly due to that high shop temprature and is the root of most of the issues you're seeing. Resin shelf life and pot life drastically drop above 80 degrees, which means you have far less time to work before the material starts to gel. When you have that little window, you're forced to rush the process, which leads directly into air entrapment. You need to mitigate that heat if you want a proper cure, perhaps by using a temporary enclosure or even circulating cool air in the shop while you work.

Regarding the bubbles, it is less about speed and more about the technique of saturation and pressure. You need to ensure the resin is thoroughly saturated into the weave of the mat before you roll it out. If the mat is just resting on the surface, it will bubble. Use a dedicated rolling technique, applying pressure along the entire length of the mat, not just in short circles. The goal is to physically squeeze out the air that gets trapped between the fiberglass fibers and the resin film. This process needs firm, consistent pressure til the mat is fully wetted out. As for the lifting at the edges, that's an adhesion problem with bare metal. Even if you've ground and cleaned the area thoroughly, the epoxy needs a bond to the substrate that is stronger than just the primer. Before laying the mat, you realy need to apply a specialized bonding agent or an adhesion promoter specifically designed for epoxy application on raw steel or aluminum. Relying solely on sanding and compressed air isn't enough when dealing with materials that have such different chemical properties. A small dab of that primer along the perimeter of the repair can lock down the mat edges and prevent them from lifting as it cures.

Forget the temperature for a minute. When I laid up my first big panel repair thirty years back, I found the real secret wasn't the air, it was the ratio. If you mix too much, it gets brittle and it cures too fast even in a cool shop. Try using a slightly thicker mix than the can calls for, maybe with a little filler, just to slow down the initial set and give you more time to get the whole thing wetted out right.

I've found that sometimes the bubbles aren't just about rolling pressure. You might want to try wetting out the fiberglass cloth completley with the resin before you even try pressing it into the patch. It sounds wierd, but if you let the mat soak a bit first, you're not fighting against the fibers soaking up the material unevenly. It really helps get those initial air pockets out before it starts to cure. What specific type of mat are you using? I've tried a few diffrent weaves for these little repairs.

The issue of trapped air often goes overlooked because people focus solely on the rolling process, but I think the sustained pressure during the initial cure phase is equally critical. Once you've achieved a thoroughly saturated weave, you really need to maintain constant, uniform pressure across the entire surface while the material is in its initial gel state to prevent the formation of microscopic voids. These tiny bubbles are often invisible immediately but coalesce into the raised spots you are observing after the epoxy has partially hardened and the material contracts. In my experience repairing a damaged tailgate, I found that using scrap plywood and clamps to weigh the patch down during the first few hours made a massive difference in the final finish.