I have never tried to adhere fiberglass to aluminum.

But...

I have painted a lot of aluminum professionally.

The problem with aluminum is that it is an extremely active metal the very top layer combines with oxygen IMMEDIATELY when exposed. This is why sanding doesn't really work.

Oh...

and when I say it combines with oxygen, what happens is it forms a layer of aluminum oxide which just doesn't adhere well to paint or glue.

To get around this you use what is called a conversion coating.

First you sand it with very rough sandpaper to provide some "tooth".

Then you use a cleaning agent to remove any grease, oil or dirt rubbed in by the sanding process.

Then you brush on the conversion coating (the stuff I used would give the aluminum a golden yellow tinge.

Then you liberally rinse off the the residue.

Let it dry.

Then apply your fiberglass as soon as possible.

Go to your local automotive paint store and ask them about it.

However...

This might not be a good idea, ie fiberglass to aluminum.

I have a feeling the coefficients of expansion will be radically different.

I ran into this problem making some signage years ago.

We had sandwiched some .40 aluminum to 3/4" plywood in cutout shapes. This was done in a relatively warm shop at about 60 degrees F. Some of the pieces were about 2 inches by 10 feet long. On the day of installation it was about 10 degrees below zero F. The long pieces that were flat in the shop had about a 5 inch bow in them after they got cold.

So what I'm saying is that if you have temperature swings the materials working against each other could cause it to come apart.

Hope I haven't confused you entirely.