GALLING
Jim, I am guessing that the screws are stainless steel and they are threaded into aluminum? Galling is common with both stainless and aluminum. The metals actually fuse together from heat caused by friction. Another word for it is "cold welding." An over tightened screw can get this way. My tonneaux cover has aluminum clamps tapped for stainless machine screws. The screws will gall with the threads if I torque them too much. I now use a little anti-seizure compound on the threads and don't tighten them so much.
Galling is usually not a huge issue in the sign industry. More an annoyance than anything, such as when a stainless sheet metal screw breaks off when you try to remove it from an aluminum cabinet. I believe that removing the screws before painting the cabinet and not over tightening the screws, or using aluminum screws are fixes.
Here are a couple of interesting links on galling:
How To Stop Thread Galling On Stainless Fasteners | Fasteners, bolts, screws and more from Atlantic Fasteners
https://www.fastenal.com/content/feds/pdf/Article - Galling.pdf
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"Gaulling" is another issue entirely and can only occur when the screws become French (I know. Lame joke. Sorry).
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GALVANIC REACTION
Another thing that can happen between stainless and aluminum is galvanic corrosion. This is when one metal corrodes and deteriorates the other. This is similar to what happens in a battery. One metal, aluminum, becomes the anode and loses electrons to the cathode, the stainless steel, in the presence of an electrolyte (moist sea air is a good electrolyte). The metals must be in direct contact for this to occur. And some stainless alloys are worse than others. In structural applications, this can be hazardous. In an indoor swimming pool environment, for example, where the air is saturated with moisture and chlorine, aluminum brackets with stainless fasteners can fail. Engineers will specify insulating washers or shims of nylon, rubber or some specially designed plastic to separate the metals to minimize the danger. Also in boat building, where stainless and aluminum are both used extensively, there are common workarounds for this problem.
Galvanic corrosion is usually not a big issue in sign making. Note the picture contained in the link below showing square aluminum tubing that was gradually destroyed near the attachment points by stainless steel through-bolts, but also note that it took a long time. In my own experience, I have seen stainless bolts ruin ACM panels attached to angle iron frames, especially the cheaper panels that have an almost paper-thin aluminum skin. I have had Polymetal, for example, develop lines of corrosion under the paint, like thin spidery fingers, radiating from the attachment point of a stainless screw. As the corrosion gets worse, it can become quite noticeable. Then the paint begins to fail in a patch around the attachment point as the aluminum virtually dissolves underneath. This may be a good argument for not using stainless screws to attach ultra thin aluminum, or for using nylon washers.
For perspective on how thin the aluminum is on ACM panels, typical is a thickness of 11 or 12 MIL, or thousandths of an inch, written as .011" and .012" respectively (MIL is not to be confused with millimeter), compared to a very heavy garbage bag which may be as much as four MIL in thickness. A typical aluminum blank for a real estate sign frame may be 40 MIL (.040"), over three times the thickness of the skin on an ACM panel.
Galvanic corrosion explained by experts:
Galvanic Corrosion – keep those metals apart! | Anzor Australia's blog - provides technical tips, case studies, and other useful information about stainless steel fasteners
SSINA: Stainless Steel: Corrosion
Square aluminum tube attacked by stainless bolts:
Aluminium Corrosion Resistance - Aluminium Design
Brad in Kansas City
