...maybe some mud and tape, like you would use for drywall? But this wouldn't work so well with a foam board, which is more flexible than the dried mud.

Other suggestion is to glue the two ends (butt-joint) of the foam boards together, then carefully sand it until it looks smooth.

Thanks @Juxen for your reply. Yes I had taped the two foam boards together and mudded/sanded them. And wasn't happy with the results like a drywall would. 

The glue end to end is a good thing to do thanks for that! I was going to just back tape them together. When I paint the foam boards after your suggestion of sanding, maybe the paint could fill the tight gap? Worth a try.

Thanks!
Brad

Here is an example of hiding a seem, directly under a light, with silos. This could be effective moving forward, but the photo is cropped. What to do with the balance of the 10" above? 

I used masking tape to join foam core behind a layout once. It's still visible once painted but it's wasn't that noticeable from a viewing distance of a couple of feet which is how wide the layout was. If I were going to really try and hide it I probably would have chosen spackling compound over the masking tape rather than drywall mud.

Yes I did the join the two foamcore with white gloss tape. And spackled it, not drywall compound. Sanded it and wasn't satisfactoy. I don't want two seems now. What do you do now? 

cheers,

Brad

This may be a good item to search on this site, as there is quite a bit of discussion on same.

I used Masonite, and may again, but would attach lineoluem to it to give me longer runs without seams. (next time...?!?). After all, how many tall smokestacks can you reasonably defend?

Like you, I fussed with the cost, so decided I would tackle it myself with paint. You might get these paints from a big box store, in quarts may be enough.

I first painted the upper 3/4 of the backdrop in a sky blue - used most of a gallon here, but it is a relatively large layout.

Then I painted in clouds by dry-brushing in wispy clouds in white - suggest you not overdo this step. Less is more.

Then I dry-brush painted the white along a horizon about 2 inches up (HO layout), feathering into the sky. This represents the hazy far horizon clouds

Then I painted in a blue/gray "far horizon" line that gave a clean line against the lowest white. It undulates just a little up and down.

Just below the blue/gray, and only an inch at most, I painted in a gray/green to represent the nearer horizon.  It sometimes was below, and sometimes above the blue/gray line, but only by a very little bit - 1/2 inch or less. Sometimes I dragged the brush up slightly to give the appearance of barely discernible treetops.

Lastly, and now with artists paints, I stippled in trees using Mars Black, Yellow and White (for birch trunks). 

In the towns, I had a bit of a time painting in the streets and far buildings, as I am too unsteady to make fine straight lines, but even clunky looking lines worked out. Here you might put a photo onto the backdrop and blend the edges with the colors mentioned above.

This photo was about the bridge going in, and the green drops much lower than the blue as I described above, because this is a river valley. But you can see the colors clearly here.

Bite the bullet.

Find a way to get some 1/8" hardboard delivered. It can be done and it's worth the effort. As to hiding the photo image seams......at least you'll have a solid flat base that won't be going anywhere.  

Hi Brad

i have wrestled with similar issue too. Tried the Masonite approach with it painted sky blue and it was ok. Certainly better than the existing purple wall with “orange peel” drywall texture.

Later I added 2 sections of commercial photo backdrop. They looked pretty good at first but I had a hard time lining them up and the seam was still visible.  Plus I find the Masonite kind of a pain to deal with.  And the photo backdrop started peeling off after a few months.  That’s probably due to my poor installation and flat paint on the Masonite more so than the photo product.  

This shows the results on the left:

All this confirmed that another approach was needed as I have about 50 feet of backdrop total. So I just tore it all down and am awaiting delivery of a 50’ roll of blue aluminum trim coil to install as backdrop.
 

Product:  https://www.homedepot.com/p/Amerimax-Home-Products-24-in-x-50-ft-Bermuda-Blue-and-White-Aluminum-Trim-Coil-69124102/205664231

Technique: https://www.nmra.org/sites/default/files/sr201202_alumback.pdf

There will be zero seams now and nicely curved corners.  I can add clouds as desired. So this is an option you might consider.  All in with J channel and punch tool less than $150 for 50’ backdrop.  

jeff

EDITED to add links and picture.  

It looks a little on the pink side here - the clouds are actually white.

The photo above showing your painted backdrop is beautiful. The ground textures and vegetation have an impressionist feel and the colors are perfect......yup, a winner!

 I'm a big fan of painted backdrops too. They've gotta be loose which is an advantage to a non trained "artist" and also allows the main subject to not be overwhelmed.

 Here's mine......a study in dry tones.