My first choice to fix low or cupped inserts is to remove the insert, sand them flat and epoxy back flush to the playfield. Vid has a great guide for doing this here:

http://pinside.com/pinball/forum/topic/vids-guide-to-ultimate-playfield-restoration

I have been testing windshield repair resin to do this repair as an alternative to sanding. There are some situations where sanding the insert flat isn’t an option:

1. Insert is glued in and removal would be a destructive
2. Insert art printed in white ink
3. Art on the insert is not easily reproduced

Having inserts firmly affixed to the playfield is important before clearcoating the playfield. If the insert is not solidly attached, over time it will move and cause cracks in the clearcoat, so I always epoxy loose inserts first. Most of the PF work I do is on early solid state Bally and Stern tables. The inserts in these games almost always can be removed with your finger.

When I’m working with inserts it is helpful to have lighting from under the playfield and a magnifying lamp. I use an excellent LED flat panel and magnifying lamp setup like this:
equipment.jpg

To start, prep the insert by cleaning with Magic Eraser and alcohol, and scuffing with 400 grit sandpaper. Once the insert is epoxied in place, clean the area with alcohol.

If the PF is not being cleared, cover the repair area with masking film and cut out the insert area with an Exacto knife. Typically, I am clearing the PF and the surface is not stable enough to use the film without lifting paint, so this step is skipped.
Mask.jpg

The windshield resin does not cure if it is not covered. This can be used as an advantage since any material not covered will stay liquid and can be wiped off the PF with a rag. With this in mind, I purchased several sizes of round Mylar for buttons used to lay over the repair area during curing.

The resin used has a viscosity close to water. Make sure the PF is leveled. Fill the prepared insert with resin and side the Mylar over the resin from the side to minimize air bubbles.
Mylar_slide.jpg

Cure the resin with a UV light. This unit uses proper UV bulbs that output UV light at 365 nm. The bottom is removable and it has both a timer switch and an on-off switch without the timer. I cure the resin from 3 to 5 minutes.
UV Cure.jpg

Here is the insert repaired with the Mylar removed and resin cured:
Cured.jpg

If you end up with unacceptable air pockets you can and fill them with resin, cover and cure like above.
rerun.jpg

Here is a repaired insert lightly sanded with 400 grit ready for clearcoat. The bubbles in the pic are on the surface and should fill with clearcoat.
Sanded.PNG

The cured resin will soften and clean up with alcohol, reacting kind of like lacquer. The next test is to confirm its compatibility with clearcoat. I will be shooting a base coat of auto clear on this playfield in the next day or two and will update this thread with the results.

Here are the products used to make the repairs:
• Alcohol for cleaning
• Windshield repair kit resin, SUPER LOW VISCOSITY (eBay)
• Clear Plastic Mylar for Button Maker Machines (eBay)
• Salon Edge 36W Nail UV Lamp (Amazon)
• Optional - Clear Masking Film (tcpglobal.com)
Insert_Products.PNG

I don't know. My first try at this I didn't cover the resin and it never setup under the UV. Cured it for an hour and it was a gooey mess. Checked with the seller and another Pinsider and they said it needed the Mylar to cure. With the cover it sets almost instantly.

Update. So far so good. The basecoat of auto clear has been sprayed and the resin looks good with no clouding.
booth.jpg

In this picture, you can see the un-sanded clear and how low the insert was. This area is absolutely flat and clear as glass. I will need to wait for the clear to setup before sanding.
basecoat.jpg

What is your application?

Sets up instantly and perfectly flat. No shrinkage.

When fixing ghosted inserts the material is covered, as in under the clear coat. The OP talks about wiping up the PF when done. I'm thinking that is uncured resin. Maybe the windshield resin is the same stuff in a different package.

I'm guessing this is the same stuff that is used in the ghosted insert post. At least in the same family. When the water thin resin runs under the clear coat it blocks the chemical interaction with oxygen, which inturn does not inhibit the curing process. I'm not a chemist or work in an industry that uses this stuff but just my observation.

The PF is ready for touchups and decals. First I leveled the inserts with resin to catch the big low spots. Then a ground coat of clear is applied and sanded with 500 grit to begin getting everything flat. Any low insert is again filled with resin. Working with the resin has been fairly straight forward. I remove the excess with little bit of alcohol sprayed on a Magic Eraser. This softens the resin. After the ME, I let the area dry and the area hardens back and looks a little cloudy. At this point sanding with 800 and 1200 grit (dry) the cloudiness is removed and insert is flat. Another heavy coat of clear was applied and sanded to get to where I am now.
playfield prepped for touchup.jpg

I am using clear water slide decals. They will go on after air brush touch ups.

I'm using auto clear. The decals are sealed after printing with an acrylic sealer to protect the ink. The first coat of clear over the decal is very light. I will post pics as I go.

Here are some more pics of filling in gaps after the base coat of clear was applied. It's not a 100% but much closer. So far the resin seems to be compatible and sands a blends in with the clear. I am air brushing the base colors now and will post more pics as thing progress.
insert1-189.jpg
insert2-175.jpg
Mylar applied:
insert3-847.jpg
After UV cure:
insert4-10.jpg
Sanded:
insert5-623.jpg

The base paint for Seawitch is on. Tomorrow it will get cleared in preparation for decals:
Paint1.jpgPaint2.jpgPaint3.jpg

I did the 4x. It wasn't cupped but a little low when I epoxied it in:
IMG_20140323_132231.jpg

No reaction at all to the auto clear. The above pic has a base coat over the resin and a coat to seal the acrylic paint. I'm installing decals right now, so once done, it will probably get a mist coat, then one or two final coats.

I've had problems with venting in the past if it's too deep. Also, when you pull the mylar up it is dead flat. When I'm spaying clear it just seems like I have to put on more coats than I want to get things flat. Also, I do early 80's stuff, so I am always dealing with a crater field.