Here's a Gottlieb Touchdown I worked on a few months back. I pulled it out of a dilapidated warehouse along with a second machine. The playfield was pretty rough--lots of wear down to the wood around the pop bumpers and the inlanes. Because of the complexity of the artwork and the time crunch of prepping it for a show, I ended up creating decals for it. So, the goal here was to end up with a decent-looking player's pin--not to do a high-end restore. There were too many other cosmetic problems with the backglass and cabinet to invest the time in doing that.
As found in the warehouse
After cleaning, but before the decals
after the decals were applied

First I stripped the playfield. Because this scanner is so large and the bezel around the scanning glass is so large, it is tricky just to scan one small area. Basically everything has to come off the playfield--including the ball guides. But, I sort of lucked out in that the ball guides that would have been in the way were broken and needed to be removed anyway. PBR had replacements available.

Wear

Next, I scanned the areas that needed attention at 600dpi. I like to scan at 600dpi since it grabs the most amount of detail as possible, and you can always scale down for printing. the downside is the filesizes can get quite large, expecially when working with these images photoshop.

The best scans were made with the lights off. This prevented color shifts, light bleed, and shadows from showing up in the scan. Scans should have about 2"-3" of overlap so that they can be merged later in photoshop. I also took high-res close-up photos of another game, which I used as a reference to re-draw the missing areas that were worn to the wood.
Scanning with an HP 4670
Scanning with the lights off

Then the scanned images were merged with photoshop using file > automate > photomerge. Select the "reposition" option so that the images don't get curved or distorted like with a set of panoramic images. If the images don't line up properly, use the "interactive layout" option and position them manually--photoshop will take care of the stiching when the photos are close enough together.
The scans stitched together using the photomerge tool in photoshop

Before getting too far with Photoshop, I did a fast-draft test print to make sure the scan lined up properly, which it did.
test printtest fitting

Then the bulk of the photoshop work...
The photoshop work completed. Ignore the jpeg compression artifacts...they don't appear when printing an uncompressed image at 300dpi
The decal areas for printing

After finishing the photoshop work, came fitting and printing everything on white waterslide decal paper with a laser printer, and adding each piece to the playfield. It is best to print from a cold start on the laser printer, rather than after it has been sitting there for a while baking. The extra heat can ruin the decal paper. Laser printers can typically cost $250-$600.

The paper comes in white or clear. Clear is generally only used for adding black text over inserts. White is generally used for everything else
http://www.decalpaper.com/SearchResults.asp?search=L825C

Printed

To prep the playfield, you can use a filler material like JB Weld and use a soft rubber edge to smooth it level with the paint on the playfield. For whatever filler you use, make sure that it is either a neutral gray, or paint over it with a neutral gray. Otherwise, it will show through the decal. For a more detailed guide on filling wear spots, a thread on The Shadow's sanctum repair may be useful: https://pinside.com/pinball/forum/topic/shadow-sanctum-repair-updated

After that, I waxed the playfield and let it sit for a few hours. Since water is used to apply the decals, this would hopefully prevent planking.

When trimming the decals, be careful not to scratch the ink with sciscors, otherwise it will come off and you will have to re-print and re-cut. I used a very tiny pair of scissors from a pocket knife to trim several of the tiny areas around the edges of black lines. The best way to hide a decal is trimming along black lines, even if the decal will cover areas that are not affected by wear. Spot repairs will be noticeable--larger areas are less likely to be noticed since there is less of a chance of color variation.

trimmed decaltrimmed decaltrimmed decal

To apply the decals, they need to be soaked in water for 30 seconds--a large bowl works fine. Then while the decal is still in the water, slide it off the paper. Take it out of the water, let some of the excess water drip off, then place the decal on the playfield and slide it around to position it. They pretty much adhere and lay flat on their own without air bubbles. Once satisfied with the positioning, dab a paper towel on and around the decal to soak up excess water. Let it dry for 24 hours. The decals will pretty much stay fixed in place unless you pick at them.

Placing the decal
Placed
Placed
One more area left...
Done!

Because these are made of white paper, wherever there is an edge, there will be white visible. After the decals dried for 24 hours, I painted the edges by hand with a tiny brush and black paint. This will take a steady hand and patience. Any mistakes can be dabbed away with a damp paper towel. Note that black on your game might not be the same true black that comes straight out of a container of paint because of age or simple color choices. It might be slightly on the gray side due to fade, or tinted slightly with another color, such as purple or red.

Next are the inlanes. This area is damaged in those particular spots because of the kick out holes in the inlanes. It's a common wear spot on this game.
wear

Since the gradient from red to yellow is actually half-tone dithering, it's a little tricky to reproduce. I ended up jumping into illustrator to create these areas.

http://shirtoid.com/tutorials/halftone-gradient-in-illustrator/

Scanned area

redrawn v2 (note--don't bother to print this since it is low-res and not to scale)

A rough trim of the decals. Note that these are a slightly different version of artwork (version 1) than depicted in the image above (version 2). This was the first revision. The reason for the change was to correct the banding in the halftone, and eliminate the thin red lines running alongside the center band with the halftone. Attempting to cut and place thin pieces of decal paper does not offer good results.
rough trim.

After placing the decals, I painted the edges by hand, like I mentioned before, and also touched up some of the wear around the inserts. For the holes for the pop bumpers, I used an exacto knife to trim the excess decal material. Then, I covered everything with mylar, and again used an exacto knife to trim the excess areas for the pop bumper holes.

I also cut an extra circle of mylar for each pop bumper area and placed that on top of the mylar that covered the width of the playfield. The idea here is to be able to replace those circles of mylar if needed at a later time, without having to pull up the mylar covering the decals and touch-ups.

painted and mylared

Done and populated

Done and populated

And here was the game as it appeared on the free play floor at Pintastic 2015:

Set up at Pintastic 2015

I included some more details above.

Waterslide details are different than vinyl cabinet decals. I didn't really encounter the bubble issue like you would see with large vinyl decals.

However, if you're considering doing a significantly large section of the playfield rather than spot repairs, you might want to consider having a vinyl overlay printed by a print shop.

For Star Trek specifically, CPR did a run of playfields not too long ago, and it looks like a few distributors like marco still have them in stock.

No, just over the decals and high wear areas.

For the pop bumpers (a high wear area), yes I added an extra layer of mylar. Years from now, if the mylar became worn out, it could be removed and replaced without disturbing the decals.

Sorry, this is the only scanner of its type that I am aware of. They still surface from time to time, though.

Sorry, don't have any suggestions. You'll have to read through printer reviews.

Waterslide decals and mylar should work fine. I don't think I've ever seen mylar get a hole worn through it, even with 30+ year old mylar.

The issue with wear on playfields is that the topcoat wasn't very durable on certain games.

Possibly Kwikwood. It is just about rock hard after it hardens. However, I normally use it to repair damage around vuk holes, rather than playfield surface damage, so another product might be more appropriate.

No. It would be about 0.08mm at most.

I would suggest starting a new thread if you want to inquire further about playfield surface repair--I'm sure others have done it and could offer some good suggestions.