Sorry for taking so long to respond but I've been a little busy and haven't even had time to check my Emails. Flash is referring to the product (Primer, Single Stage Paint, and Clear Coat) that has been applied to your vehicle, when a product has flashed it means that sufficient solvents have evaporated so that it almost feels dry to the touch. When you can touch the sprayed surface (and I usually touch the masked area, not the sprayed surface) and it's not wet, it has flashed off and your ready for your next coat. It can feel tacky but not completely set up with primers single stage and clear coats, base coat is flashed when it appears to be dry and won't tape mark if you put a piece of masking tape on it. When base coat has reached this stage, it can be clear coated. Flash times are one of the most important components in a quality paint job. If you don't allow proper flash times, solvents can be trapped resulting in sinking, solvent popping and in base coat clear coat applications result in the delamination of the top coat (clear coat). Typically flash times are 15 to 20 minutes depending on the product, what speed of hardner and or reducer you use, air flow and the ambient air temperature. For example if your spraying and epoxy primer using a medium hardner and medium reducer, the temperature that your spraying at is about 70 degrees and you have fans clearing out the air in your painting environment, your applying the primer at a medium wet rate, flash times would be 15 to 20 minutes. If any one of these variables changes, speed of hardners & reducers, air temperature and air flow, the flash times can be faster or slower. Feel your masked area to make sure that the product is setting up, tacky...not wet.
As I mentioned feel the masked area (I use the back of my hand and gently brush it over the area, if I can brush my hand over the primed area and it doesn't mark the primed area and I've waited the appropriate time it has flashed)...Base Coat needs to be dry before it's considered flashed, one of the biggest mistakes painters make is to not allow enough flash time...I have found that if I feel I've done everything correctly, medium wet coats, reducers, hardners, temperature, air flow and I feel the area and it feels flashed...give it another 5 to 10 minutes for insurance, then you know it's flashed.
Leave your masking paper and tape on the vehicle between coat of the same priming session, no need to re-mask between coats.
If you need or would want a more detailed and technical reason for flash times I'd be happy to give it to you. It's a rather long drawn out process to explain what solvents do after they are applied...but when explained many painters seem to understand the importance more readily.