How many here scratch build using foam core board ? What paint is best to use on foam core board?

I use foam core for the framing or shell, black foam core is best.  Then I cover it with a siding or plastic material like plastruct sheets.

i don't paint the foam core itself, it tends to warp.

Not sure what you should use, but I can tell you to be careful using spray paint on it.  The carrier solvent can eat away the foam between the paper facing.

I've painted it with rust-oleum spray paint ant it worked very well. I found 1/8 inch foam core at Blick Art and that works really well for curbs

My grand kids painted some things I made them from foam board, they warped too. I don't remember what kind of paint they used, but it would have been for kids, they were small at the time. I like Rich's idea of covering it with Plastruct, that sounds like a good idea to me. 

There is something called Gator foam or Gator board that I believe is supposed to be paintable without warping, but I've not yet tried any of it. For that matter I don't recall ever seeing any in person either? Saw an add for it somewhere a while back, but I'm sorry I don't remember where it was. Google search maybe, if you are interested.

Micro Mark has Gatorboard. You have to paint both sides of foam board to stop the warping.

+1 on painting both sides.  The back doesn't have to be pretty... 

Good idea!....Architects have been mocking up structures for clients using foamcore board for years. 

Some comments to add to the above responses...

Similar to Rich's idea of laminating Plastruct to the foamcore shell, I've seen several nice 'temporary' structures on layouts done using printed paper....wall textures/materials, windows, doors, etc., etc....laminated to the shell with spray adhesive (NOT white glue....which will induce warpage).  Evans Design, Clever, et al, have structural design programs for your computer which can be printed, cut out, and laminated.

Gatorboard is a nice, non-warping structural material, but it's not that easy to work.  The wood composite outer layers are not  easily cut using a hobby knife...a saw is pretty much necessary. 

MicroMark is also selling a new material called TaskBoard..."The Architects' Choice"...which carries this description: "Taskboard is an ultra-lightweight, rigid wood-fiberboard that's easy to cut with a knife and especially good to cut with a laser. Cut edges are smooth and require minimal sanding. Taskboard comes in a flat sheet, but the 1/32 inch and 1/16 inch thicknesses can be easily formed into 3D shapes . . . just spray with water, form to shape and hold until dry. The white Taskboard has a finish that readily accepts glue and paint."  We (LHS) have just started to carry it, have not tried it ourselves, but it appears to be a nicely improved compromise to foamcore and Gatorboard.  (Available sheet size may be an issue for some projects, however.)

Re painting foamcore....    I've personally found that inducing warpage to the shell material can be minimized if the paint is applied thinly (sprayed, preferably) with each coat immediately dried with a pistol-type hairdryer.   (I made a 'holster' for my dryer, attached it to the side of the spray booth.)  And, of course, solvent based paints should induce less warping, regardless, than acrylics (water-based).

Finally, foamcore pieces glued at 90-degrees to the backside of walls, or complete interior walls, can be used to reinforce  outer walls against warpage.  Of course scrap, sturdy stripwood will also work for reinforcement, too.

I've seen some pretty 'incredibly credible' structures using foamcore.  Some of those so-called mock-ups are worthy of permanence.......IMHO.

"Lucas Gudinov"

I have built structures using generic foam core board and painting the surface with 2 or 3 very light (to minimize warp) coats of acrylic paint.

Two hints on working with foam core:

1) You need a VERY sharp knife to do proper cuts. X-acto blades are fine except they will only be good for a few cuts and they are very expensive. Instead, use single-edge razor blades. You can get a box of 100 for just a few dollars. Don't be a hoarder like me--use them once or twice and throw them away. This is the secret to good foam-core work.

2) Hot glue works extremely well with foam core. Running a bead along the inside of right-angle joints is quick, rigid, and permanent.  Best to use low-temperature glue--it is possible to melt the foam. Use the glue gun like a caulk gun: PUSH the glue ahead of the nozzle to get a nice, smooth bead.

Foam core board is fine for areas that don't show, I suppose. But it often frays when cut and will warp when painted. Gatorboard on the other hand, is foam core board with a thin outer shell that won't warp, is easy to cut and takes paint easily as well. It's also structurally very rigid so it can be used for buildings, supports and anything else you can think of. You can Google it and get different sizes easily. I also use Taskboard. Very interesting material. I often use for roads as it has a very slight texture to the surface, rather than being purely smooth. It also cuts easily and takes paint well but it can warp. It's easy to get rid of the warping by weighting it down for awhile. Sometimes, smaller pieces will just flatten out by themselves. You can get very large sizes of Taskboard at Dick Blick's online. 

dkdkrd posted:

 

Finally, foamcore pieces glued at 90-degrees to the backside of walls, or complete interior walls, can be used to reinforce  outer walls against warpage.  Of course scrap, sturdy stripwood will also work for reinforcement, too.

 

That's right, I remember that now. On the stuff I made for the grand kids (buildings for stuffed animals, fairly large) I reinforced the the corners of the square parts of the building with strip wood. It was the stuff they have at Hobby Lobby/Michaels and maybe like 3/8"x3/8" or so (been a while). Anyway the pieces that were reinforced were barely (if any) warped. They were also glued solidly to a bottom floor piece. It was the removable roofs that really had the warping, not nearly as much reinforcing there.

I hadn't heard of Taskboard either, will check that out, Thanks.

Have you looked at using sign board? (coroplast)

It's rigid and takes paint well. Fairly easy to cut. Has hollow chambers though like cardboard. Holds up to moisture too.

Thanks for the idea Joe. I have 2 sheets of that at 3'X4'. Now to think of something to build! LOL

I have used foam board many times. No issues with warping. It all comes down to bracing and overall structure design. I build mine with a initial plan of how it will be constructed and use all rabbit joints and a ton of interior bracing. I also paint it with spray paint. Only one side. Very light mist coats. Seal all edges with foam paste and acrylic texture paste. I also remove sections of the paper and carve brick detail and such. I am at work now. But when I get home I will show some pictures of how I brace my structures. And provide any other details you would like to know. 

I intend to lay 1/2"  foamboard  sheets flat on top of the homasote/plywood base to raise the town level on a 26"x80"inch section.  Since it is not being used to make buildings is it necessary to paint both sides? Also have black version for rail yard and streets. I am presently using Brennan's book on making realistic scenery as an guide for these projects.

No. Just glue one side down with some liquid nails. Then paint the upper surface to seal the board and you will be fine. I have done it and used white glue etc to add scenery over the top with no issues when it's sealed. 

I love working with Foam Core and agree it does tend to warp when painted.  Here are two Foam Core buildings I completed this year and placed on the TMB club layout. The brick face building had wood slats and some wood strips behind to minimize the warp. The steel mill was used to show trains coming in/out of the factory rather than a another portal. There I glued 1x2" strips of manila folder that were crimped to look like corrugated metal. Then it was painted and weathered, NO warping here.

BTW...  There are some neat how-to's on YouTube, etc., if you search Foam Core Modeling. 

One such shows the basics of building an architectural model....in which he uses Foamwerks tools...

Foamwerks link

...which are really cool, well thought out, if you're going to do a lot of this sort of building.

And, here's another interesting site that focuses on this modeling material...

Foam Core Model Link

FWIW, of course...

"Lucas Gudinov"

I use Precision Board, (see How to work with Precision Board, Garden Railways Aug 2007). You can cut, sand and carve it just like wood. it takes any kind of paint and will not warp. I get scraps from a sign shop and use wood working tools to cut and shape it. There are suppliers that will sell small quantities. (Rainbowridgekits.com, www.precissionboard.com)