If you look closely at your playfield your are going to see thousands of tiny scratches. The majority of those scratches were caused by improper cleaning. So when cleaning a playfield we have two goals: get the game clean plus minimize adding any more scratches (which can only be removed through polishing).
The first question is what type of cleaning you're doing. If you just picked up the game and it's filthy I would completely strip the playfield and then clean, polish (if necessary) and wax (see the references for polishing and waxing). At the same time you can clean the plastics and replace the rubbers and any dead bulbs.
On the other hand if it's just a six month tune-up, I would remove enough stuff to get to the playfield and do a good cleaning and waxing.
In either case, leave the Novus in your toolbox. While great for plastics they are not the best mix for playfield cleaning (or polishing). There is not any high-end restorer that uses Novus products to clean or polish playfields (see this article by Classic Playfield Reproductions on buffing playfields and here's an article comparing the Treasure Cover Polishing Kit to Novus).
Thoroughly cleaning a pinball playfield is a three, and possibly four, step process.
In Image 1 you can see a fairly typical playfield that hasn't been cleaned in years. Note the blackening in the yellow areas and the ball marks just to the left of the babe in the flying bridge and in the outlanes.
Before starting use a shop-vac (or air compressor) and an old paint brush to remove any loose dirt. A good portion of that black stuff is actually metallic grit that will scratch the playfield if rubbed in.
While a lot of people use Novus 1 or Simple Green, I prefer a citrus based cleaner like ZEP Heavy Duty Citrus Degreaser (available at Home Depot) which will also remove old wax. On the other hand if you don't want to remove the old wax, use Simple Green.
On older games, test any cleaner on a small inconspicuous area first.
See the references for more info on pinball cleaning products. One product I will specifically mention is Mill Wax pinball cleaner/wax. There are a lot of discussions on the web regarding the potential damage Mill Wax can do to modern pinballs. Rather than enter into that discussion I will just say a combo cleaner and wax is really not very good at either job.
Always spray the cleaner onto your towel, not directly onto the playfield. Water is a pinball games worst enemy, so we want to limit how much, and where, the liquid goes.
Depending on the size of your towel, fold it in half and then in half again. As you're cleaning switch to a new clean side as the towel gets dirty. Again, this is to prevent scratching the finish with the metallic grit.
Secondly I always check my towel every so often just to make sure I'm not seeing any color transfer from the paint. Not typically a big issue on playfields, but still a good habit to get into. This is more important the older the game is.
At this point I'm sure you know what to do. Keep cleaning each section until the towel comes away clean. Then wipe down with a dry, clean towel. In Image 2 you can see the playfield after cleaning. The yellow areas are still blackened and the ball marks are still there, although better.
See Images 4-6 for before and after cleaning photos from an Addams Family.
Automotive pre-wax cleaners are designed to remove contaminants that normal cleaning leaves behind. They also do an unbelievable job on pinball playfields and most are non-abrasive (see the references for more info).
For enamel and lacquer finishes Collinite Liquid Sapphire PreWax Cleaner is a non-abrasive cleaner that will remove ground in dirt, film, discoloration and wax stains. For playfields with automotive clear coat Pinnacle Paintwork Cleansing Solution is a great choice. It uses a combination of gloss-enhancing oils and micro-fine fillers to polish without abrasives.
Both products are designed to remove contaminants that normal cleaning does not remove. They are applied by hand, left to dry to a haze (it doesn't take long) and then buffed out by hand or with an orbital buffer (preferred method). The Pinnacle product works best when used with a white polishing pad (see references).
I've had situations where I was planning on polishing the playfield but after using these products found it either wasn't necessary or only a quick pass with 3M Finesse-It Machine Polish was required. The pre-wax cleaner will also remove a lot of ground in grit and in many cases preclude moving on to more aggressive approaches.
See Images 7-10 for before and after photos on an Addams Family using the Pinnacle product. The game was thoroughly cleaned prior to using the pre-wax cleaner.
In Image 7 all of the black grit has been removed although there are some dimples in the playfield that cast dark shadows. I'll fix the dimples later with clear coat scratch remover.
In Image 9 the ball track covered the gray area from top to bottom and at this point I have cleaned only the lower circled area to provide a comparison. While the pre-wax cleaner did not get all of the grit out, it got most of it out which will minimize the requirement for more aggressive steps. Although based on past experience once I go to the orbital buffer this area should clean up even better.
In Image 10 there's a little grit left, but once removed from the playfield I'm sure the pre-wax cleaner will get the remaining spots.
In Image 11 you can see the gloss that I'm starting to get without polishing or waxing (unfortunately there's also some orange peel, but I'll get to that in the polishing article).
I should also note that I did this by hand rather than with an orbital buffer and white pad, which gives better results. After I strip the playfield I'll do it the right way, but for the sake of completing this article I got ahead of myself.
Removing Ball Marks and Embedded Grit
First I should discuss Novus 2 since it is such a popular recommendation for cleaning dirty playfields. Novus 2 is a polish and according to Novus is equivalent to 1200 grit wet/dry sandpaper. While any abrasive will remove ball marks and embedded grit, there are much better, and safer options.
In fact at this point we could move on to polishing, but instead we'll use a Magic Eraser to lift it out. Although mildly abrasive, this solution is still much less so than Novus 2 and better than grinding the grit around the playfield with an orbital buffer.
Magic Eraser's are made from melamine foam and while I haven't been able to identify a specific grit, the general consensus is somewhere between 2,000 and 3,000. The big advantage though is that melamine has a structure like fine fiberglass strands and will get down into cracks that other methods won't. Bottom line, it is less abrasive than Novus 2 and requires less rubbing, which means less abrasion, thanks to its structure.
The other reason for using the ME with alcohol is the grit will be lifted away from the playfield. On the other hand a polishing compound will just rub the grit around and cause further scratches in the finish.
It is abrasive though so it should be used with caution. Also be aware you will often have to polish the area after using the Magic Eraser.
Lightly moisten the ME with the alcohol. It does not take much and you do not want the mixture of cleaner and worn off melamine foam running or dripping into unintended areas. It is a pain to clean up and should be wiped up frequently or the alcohol will soften the playfield paint.
There is no reason to use the eraser on the entire playfield, just on selected areas.
A small area (6" by 6") should not take more than ten to twenty seconds to clean. If it's not clean by then you're probably not going to get it clean and risk removing paint. Frequently check the ME for paint transfer and stop if you see any. Unfortunately this won't work with clear-coat, but if you follow my time rule you'll be fine.
There are occasions where the Magic Eraser and alcohol will leave a haze and/or dull the playfield finish. If that happens just use some Naptha to remove the haze and then polish the area with a high quality finishing polish.
See Image 3 for the final product. It took me about fifteen seconds to get the yellow area between the bumpers clean with a Magic Eraser and about 10 seconds to remove the ball marks to the left of the girl on the bridge. I forgot to get a photo after I did the outlanes, but they came out perfectly and took about 10 seconds each.