In my experience the thin layer of original clear contains dirt and is yellowing. For me that are two reasons to remove it. As the playfield will be clearcoated anyway there is no point in trying to keep the original clear. I use nitro thinner to remove it, applied on a cloth. Gently rubbing to remove the yellowed layer of varnish, it is more efficient than magic eraser and safer than sandpaper.

As an example: it took me about 10 seconds to clean up the "10" below, removing the old clear and ball swirl dirt.

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This is the product I use: http://www.resolve.de/fileadmin/user_upload/resolve/downloads/datenblaetter_oeffentlichkeit/Technische_Merkblaetter/Staufen/staufen_tm_nitroverduennung_-_10_2016.pdf

Google translates the specifics as such:
High-quality fresh produce thinner for nitro, nitro combination, air drying and burn-in synthetic resin paints; Removes dirt, grease and paint stains from wood and stone floors, glass etc.

I stumbled upon it by accident. But since then it's the only product I use to clean up playfields before restorations.

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@arqpuebla: Nice project so far. Following with great attention. I noticed you cut the mylar into strips. Great minds think alike , I did the same thing myself lately. Never saw anybody do that before. But it's good to share this because indeed it's much easier and safe to remove these strips, than dealing with the full size piece of mylar.

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