These are signs of non-filtered air entering the booth through leaks. Caulking periodically needs to be inspected. Scrape away loose caulking and apply new as needed. Wash away the streaks and watch to see if they return or not.
The spray booth pressure is operating too high. If the pressure is maintained between .01” and .05” W.C positive, streaks will not form. If you have a manually adjusted system, one that the painter has to look at a gauge and turn a knob or handle or switch to adjust a damper position or motor speed, then consider an upgrade to a system that will automatically maintain the pressure.
This could be because there is not adequate fresh air introduced during the bake cycle, low baking temperature, or low air flow. Consider having our heating and air flow analysis completed to determine the cause.
A common cause to this is build up of paint on the fan blades. This causes the fan to run off balance. Typically removing the fan blade and either power washing it or even having it sandblasted will take care of this. In some cases they can be power washed while installed. There is a risk in not getting it all off the blades and it remaining off balance.
Check with your paint supplier for the type of coating you are using. Keep in mind that most paint companies are looking for certain panel temperatures not necessarily air temperature. The air temperature should be set at about 20 degrees higher than the desired panel temperature.
Density. Fiberglass is less dense, allowing for a much higher particulate count down stream of the filter. This will result in a faster build up in the ductwork, on the fan blades and worse yet if your spray booth re-circulates during the bake cycle when these particulates will accumulate on the burner and ceiling filters as well.
A good way to look at this question is to use an analogy of an electric stove vs. a gas stove. When you turn the burner on in an electric stove you can actually place your hand on the burner (though we don’t recommend this!) and not burn your hand for the first several seconds. On a gas stove as soon as you turn the burner on it is too hot to get close to it even! That’s exactly how direct and indirect fired burners work. The in-direct fired systems uses a very large burner and heats up a large chamber that has to get extremely hot so that it can warm up the air that passes over it before it goes into the spray booth. The direct fired system uses about the same size burner but the air passes directly over the flame before going in to the booth.