I thought it might be helpful to add my Firepower Hardtop experience to this thread.
First I thought the process would go pretty much how you did it here, but then I made a detour.

After looking closely at the playfield in my complete Firepower, I could not bring myself to sand off the artwork.
There inserts were cupped, but the only artwork that needs touched up is pretty much all blacks.
So, I will not sand this one, I found a donor playfield that was beyond restoration.

Now this simple Hardtop install turns into a full playfield swap.
I do not think that really adds too much work since I always take off all the mechs and refurb those anyway.

0 Too Good to Sand.JPG

This is the Donor playfield, thanks to SNYPER2009

1 Donor Firepower.JPG

First I stripped everything off even the Mylar.

2 Mylar Off.JPG

Next I sanded the back side.

3 Backside sanded (resized).JPG

I decided to keep the old inserts, lots of cleaning and sanding.

4 Ready to sand (resized).JPG

Now the artwork is all sanded off.

5 Sanded Inserts out (resized).JPG

Next I glued the inserts back in.

6 Inserts Glued (resized).JPG

After sanding down to 320 grit I sprayed Rustoleum 2X gloss clear on the inserts for a nice shine.
I also blacked around the roll over switch slots since they show after the Hardtop is on.

7 Cleared and Blacked switches (resized).JPG

Now is the time to clamp the Hardtop down and stick it.
(2nd set of hands thanks to my wife for holding the Hardtop up while I pealed off the backing)

8 Ready to stick (resized).JPG

It is done, the Hardtop went down and the inserts are aligned. (inserts are all that really matters)

9 Stuck On (resized).JPG

Next was the routing out of the Star roll overs, slowly and carefully.
Then a quick coat of wax.

10 Rollovers and Wax (resized).JPG

Now on to the playfield swap.
I will have a pretty nice restore-able Firepower playfield left over when this is done.

This is my version of the Firepower Hardtop experience, I am feeling really good about it.
I still have a ways to go before I can play it.

I just followed Vids guide, not much heat required on my inserts.

I did not want to order new and wait for them to arrive, I am impatient.

They all seemed clean and undamaged.

Damn the torpedos!
Keep calm and carry on.
Don't worry be happy.
Any every other cliché that encourages keeping going.