Silicone spray can lead to adhesion issues if painting is involved in the future.
Another option to consider for equalizing the blotchy surface and incidental removal of some remaining coating is to use full strength blue a/c condensing coil cleaner. It is a caustic product in terms of pH that is specifically formulated to clean and brighten (attack..) the aluminum fins to benefit heat transfer. Apply, let rest and rinse off as instructed. Repeat as needed. It’s around $60/gallon for full strength pro stuff by the single jug unless you buy it in bulk by case or pallet as a pro, where it only costs around $20/gallon. So find a pro that’s a friend…. If you can find the diluted consumer squirt bottles, it’s largely useless imho. A friend that restores old HD bikes and carbs uses it and it can produce great results.
I’ve found that the first few rounds don’t seem to do much. Be patient, do several application cycles. Even though it’s attacking the metal, it’s a slow process when done correctly. the change between each application may not be dramatic but the before and after pics of the whole process can show much more obvious changes.
It doesn’t disrupt the surface like blasting or abrasion can, so for pretty castings it’s a nice way to make them look new again.
