Hmmm.... You say an inch wide by an inch deep, so assuming these are not through-holes. Those would be a little harder to patch, especially getting everything to 'stick'.

Also, you also say these holes were made by anchors - so is this more of a structural repair vs cosmetic?

At first glance, spray foam seems to be the best and would most closely match the original foam material. Of course if you only have a few small holes, once you start using the can, it would likely go bad over time. So might waste nearly a full can for a few small holes. it would also be messy and need several steps to clean-up and shape to final form. If you do go this route, you might try the 'low expanding' window/door type. It seems to be a bit more dense and would probably be more durable than the 'expanding' foam which seems to be more...well...foamy!

Couple other possibilities that come to mind... a paste of elmer's glue and saw dust, "Bondo" body filler, "monster mud" molding compound, drywall joint compound, play-dough. Possibly others, but I guess anything that has a pasty/sticky consistency that will 'set up' to some solid form.

One thing to be careful of - at 1 x 1 inch - if you use anything that needs to physically 'dry' vs 'chemically set', you might apply in a few thin layers with dry time between each - or the center could stay gooey for a long time - and possibly the whole mass could shrink and crack as it dries.

 

You can lay parchment paper over foam spray and pre shape it. The foam won't stick to the parchment paper and air will pass through, so it'll still cure quickly.

 

How thick are the tombstones? It sounds like the outer walls of the foam where you have the PVC embedded aren't thick enough. Then when the wind blows against the stones, most of that shearing force is against the rest of the stone, where the only resistance point is the short pieces of PVC at the bottom end. That pivot point isn't strong enough to resist that force when the PVC is only an inch long.

I have several suggestions.

Use longer PVC. The 1" deep ones you have aren't sufficient. This will help spread that shearing force along the stone instead of concentrating it at the bottom. For my stones, I embed two pieces of PVC, 8" long, spaced evenly apart. I drive two piece of rebar into the ground at least 6" deep, and keep between 6" and 7" of it above ground to slide the stone down onto. I drive the rebar in at slight angles away from each other. This creates a little tension that helps hold the tombstone in place.

Create bases for your tombstones. This will not only make the bottom thicker and harder to tear out, but also help stabilize the stone when it is windy out, spreading the shearing force out from the tombstone a bit.

As a suggestion, cut another piece of foam in the shape of your tombstone, and carve channels into it and your old stones with the intent that you will glue them together to make the tombstone thicker. That way you don't have just a thin piece of foam between the edge of the outer faces and the edge of the PVC. The combination of longer PVC and thicker foam between the PVC and the outside faces of the stone will prevent the shearing force of wind from tearing it out again.

I make every tombstone that I sell this way, and have yet to lose one or have one damaged due to high winds. We usually get 40+ mph winds here around Halloween, and my tombstones stay put and remain undamaged.

 

Do you have any scrap foam you could glue in the holes? Have you ever heard of foam clay? I haven't used it but it might work for patching. But I agree onother ideas to help this from happening again.

 

If it were me, I'd probably try mixing plaster of Paris with drylock to make a durable putty, and then touch up the paint job with more drylock, of course.

 

If they are in the back and not seen, spray foam low expansion. If they are in the front, then a bit of monster mud to patch and texture and then paint. If the damage still shows, just make it look like it belongs there. If your not working with at least 2 layers of foam ( like the store bought ones for example are 1 layer) then it’s to thin for windy areas. You can easily add extra to the backside.