Chris,
You have a less than complete description here, but I will run along it
as best I can.

SS in AL can well be electrolytic corrosion. If it is, you have a
problem.

The first try should always be to heat the AL as its coefficient of
thermal expansion is almost twice that of SS. heat it as much as you
can. If the fastener does not come loose while it is hot, then cool it
with penetrant (the kroil mentioned by another is good if you can find
it. This will cause the penetrant to be drawn into the assembly.

The SS in steel, can not be helped much with heat that does not bring
parts to incandecense and that is usually bad around plastics.

As to removing the plexi stuck in with a silicone based compound, there
not much you can do that is short of major distruction. If you can work
an Xacto blade into the joint, you might be able to make it let go, but
that is about it.

Good luck
Matt Colie

Chris wrote:
Hi maybe you can help....three of the screws is frozen on the traveler
Philips head. What's best heating and if so do I use a heat gun on the
screw head? The traveler is aluminum. I also have a deck organizer
Schafer with frozen flat screw heads into a iron embedded backing
plate.
Question 2 what is the best way to remove plastic hatch covers from
their aluminum Frame also heating the Silicone glue
And #3 Anybody have a place in Miami Fl for Lexan grey tint hatch
replacements?
Thanks in advance Chris