I've done my share of cleaning up aluminum.
Start w/ 300 grit wet-dry and work down to 1500 grit (don't cheat, use all grits in-between finishing off w/ steel wool and a metal polish.
I have never seal aluminum afterwards since the 'sealant' has a habit of changing colors and flaking off over time. Just keep polish the aluminum frequently.
This is ALLOT of work!!!!! I can understand why shops do it for $1,000+, but I'ld be very wary of the $100 shop.
As alway, if it sounds to good to be true, it probably is.
1977 Yamaha RD 400 French Blue
1977 Yamaha RD 400 Chappy Red
1984 Suzuki Gamma