Not allowed ?
Odd

Powder coating does indead work on aluminum. But it requires a different cleaning process than steel.

Some powder coaters will only have 1 or the other pre cleaning systems. Make sure the places you check have the ability to treat aluminum.

If the frame was previously coated with wet paint, it needs to be completly removed. Every speck. If it isnt, the old paint will bubble up in the oven and ruin the new finish.