Brush-On Low-Expansion Polyurethane Foam?

[Ed Kapuscinski]

Agree. Why bother with anything else than latex paint? You will add scenery material on top anyway, won't you? The small gaps and marks you want to cover will not be noticable under that.

I must disagree with you, especially in N scale. If you don't start with a smooth base, you're going to have a moon crater scene. I'm not saying it needs a mirror finish, but carving foam often leaves plenty of artifacts, especially when you have to "dig into it" with a knife. The top coat goes a LONG way toward giving you a good base to start from. I mean, remember a tiny crater could eat an N scale man.

Also, @Sokramiketes, I can't imagine how much time it would take to carve foam smoothly enough to leave no trace, but I have other things to do with my life...

[Sokramiketes]

I must disagree with you, especially in N scale. If you don't start with a smooth base, you're going to have a moon crater scene. I'm not saying it needs a mirror finish, but carving foam often leaves plenty of artifacts, especially when you have to "dig into it" with a knife. The top coat goes a LONG way toward giving you a good base to start from. I mean, remember a tiny crater could eat an N scale man.

Also, @Sokramiketes, I can't imagine how much time it would take to carve foam smoothly enough to leave no trace, but I have other things to do with my life...

I think you're picturing the surface left by a Surfoam planer (torn out craters) and I'm picturing the surface of carved foam after sanding it with a little 100 grit after the rough carving.  I think you'd be surprised how well a coat of tan latex paint and initial sprinkle of ground foam can be for evening out the surface. 

Here's an early shot of a Modutrak corner showing the surface finish prior to paint.


And here it is after scenery.  On the left of the road is still just WS Green Blend fine turf into the wet latex paint.  The right of the road is after static grass.


I dislike topcoats (plaster, Sculptamold, etc.) because it makes it harder to plant poles and trees.  Latex paint over foam is easy to puncture without a tool.  I'll still use some spackle at joints, but not over the entire surface.

I guess I just can't see expanding polyurethane foam of any sort providing an even surface finish.  It doesn't expand consistently. 

Mike-  Why don't you start with Gorilla Glue.  Its expanding foam and comes in small test bottles already...  Think it is even your 3x range of expansion.

[Scottl]

I'm from the school of thought that irregular surfaces are both realistic and more easily obtained from raw model surfaces like foam.  Surfoam planing has few natural processes as an analog.  Heavy equipment produces these surfaces to some extent, but many modeled landscapes look more like parks than the typical natural or disused adjacent areas you often see near the tracks.

Some of the best surfaces I have produced have been paint over roughed foam (even beaded EPS) and covered with grout, a little foam and other texture finishes like static grass.

I've also used expanding foam to fill gaps and even did an entire mountain out of it on my current layout.  It has many advantages in the former application as @Sokramiketes illustrates, but I remain unconvinced that it is easier for surfaces because it is hard to dispense for that purpose.

[svedblen]

I think you're picturing the surface left by a Surfoam planer (torn out craters) and I'm picturing the surface of carved foam after sanding it with a little 100 grit after the rough carving.  I think you'd be surprised how well a coat of tan latex paint and initial sprinkle of ground foam can be for evening out the surface. 

Hear, hear!

I dislike topcoats (plaster, Sculptamold, etc.) because it makes it harder to plant poles and trees.  Latex paint over foam is easy to puncture without a tool. 

Exactly!

I'll still use some spackle at joints, but not over the entire surface.

Very wise. I do the same 

[SandyEggoJake]

I dislike topcoats (plaster, Sculptamold, etc.) because it makes it harder to plant poles and trees.

Actually, while I don't like plaster type topcoats for other reasons, having a hard surface has a lot of benefits for planting poles and trees.  Sure, it takes more effort to make the planting hole, and might require a tool to cut.  So I can see your point for a static layout.  But in modules that get knocked about?  Having a deeper / harder surface provides more fixture to such items.  And I'd rather fix a bent tree or a broken pole than needing to come back and fill then re-scenic the surface after the inevitable accident.

[Mark W]

Personally, I've used a spackle & latex mix too.  Yep, thin layers are OK and for home layouts too.  But even that is on the brittle side and inevitably cracks when used as a filler say in modular layouts that get knocked around in transport.    Yes, the mix is certainly better than spackle alone as it's more flexible.  I too would like a better, more flexible and more compatible options for a skim coat over extruded polystyrene.

I'm not sure we're on the same page.  Spackle is quite flexible itself.  If you're getting cracks, even with modules, you're going beyond filling holes and finishing surfaces.  Use Great Stuff to 'glue' all sub-terrain foam material in place, carve and shape as desired, then fill/smooth with Spackle, and finish with latex. 

[C855B]

Never mind. See mea culpa below.

Testing completed tonight on several of the suggestions, and we have a winner:



This is Smooth-On FOAM-iT! 10 SLOW. It is pretty much what I was after - a slow-expanding urethane foam that could be brushed over pink foam, curing close to the same density and toolability. It could be worked with the brush (cheap foam brush in this case) while it was expanding, which is what allowed me to form the approximate profile of what will eventually be a riverbank. You can see it rose to about an inch at the back. (Yeah, much more carving ahead.). Crust developed in about an hour, but it's going to be a day or so before it's cured enough to tool due to the "slow" formula. @Scottl 's reference was the lead; I bought a trial kit directly from Smooth-On's retail arm, Reynolds Advanced Materials.

This task was going to be too much volume for lightweight spackling, which I still like for joints and small gap filling. Thinning the spackling, while sort of successful, really didn't gain much over scooping fresh stuff out of the pail with a putty knife.

@Sokramiketes , I tried your inspired Gorilla Glue idea. It rose modestly as was desired, but wasn't workable during expansion, and cured surprisingly hard all the way through. Reminded me of extra-thick urethane floor finish, but not transparent.

Another medium I had on-hand and used before on other projects was heavy-bodied artist's acrylic paint. I filled a gap to gauge the shrinkage, and it shrunk considerably. I might use it (available in quart buckets!) for surfacing, but it can't be used the same way the foam worked. It's also a bit on the dense side, like latex caulk.

[davefoxx]

Mike,

First of all, please do what works for you.  This post is not meant to be construed in any way as I am telling you what to do on your layout or that my way is better.

That said, I don't understand the benefits of this "brushable" foam.  In the time that you wait for this to expand (seemingly without 100% control), I would have carved the river's edge out of the 2" foam and smoothed it with lightweight spackling.  The spackling would set up much quicker than the slow-expanding urethane foam, and I'd be moving on.  Not to mention that the spackling requires no mixing and, because I can merely close the lid, there's no waste.  What's the advantage of your method?  It seems not only slower but messier.   

DFF

[C855B]

I'm working to avoid the large amount of carving and piecing-together of foam scraps for geoforming. The expanding foam fits my work style better. Bear in mind the layout work is not organized serially, there is no time loss in waiting for curing, I move on to something else and then come back later to [whatever] task needed the curing time. I'm also looking forward to trying rock molds with it.

As you say, YMMV.

[Mark W]

I'm working to avoid the large amount of carving and piecing-together of foam scraps for geoforming.

It's interesting, I've found that by specifically piecing-together foam scraps, my terrain results in a more natural finish ...  (Example: Bean Mountain - http://imgur.com/a/HaiW4)

Like Dave, I don't yet see the benefits of this approach, but glad you are at least giving it a try.  I'm looking forward to see the rest of this workflow unfold to compare results.

[Scottl]

Cool!  I have had success using the spray foam as an adhesive for rock castings.  It is very grippy and you just need to keep your hands clean while you do it.

I will be interested to see how easy it is to plant trees and such in the cured product.  Is it a 1:1 volume mix?

[davefoxx]

I'm working to avoid the large amount of carving and piecing-together of foam scraps for geoforming. The expanding foam fits my work style better. Bear in mind the layout work is not organized serially, there is no time loss in waiting for curing, I move on to something else and then come back later to [whatever] task needed the curing time.

Gotcha.  Carry on!

I'm also looking forward to trying rock molds with it.

Interesting!  Please let us know the results of this experiment.

DFF

[C855B]

Mea culpa. Well, not entirely, but it is new information which cancels much of the original assessment from last night.

FOAM-iT! 10 SLOW cures rock-hard. You can hammer nails with this stuff. It's not going to be reasonably workable with normal foam forming tools like Surform planes and rasps. It would be workable with power tools, but the density was closer to hardwoods. Not a good match to pink foam. Thankfully, I was able to peel it off the pink foam test site with a putty knife, in one very solid block. I'm sure this product probably has an application in MRR scenery (the aforementioned rock molds), but as a contouring helper, not so much.

I'm going to sleep on this. Their least-dense product, FOAM-iT! 3, might work, but it has a very short open time - 4 to 5 minutes. Still might be a better situation than wrestling with Great Stuff.

[Scottl]

A lot of polymers have properties that can be altered.  Can you mix in anything to soften the cure?  I know with the resin we use, mixing in acetone gives it a much softer cure, almost rubbery, compared to the normal product that rock hard (and we grind our final castings). 

[pdx1955]

I've been thinking about this method - I can see why you want to do this but I'm having a hard time thinking that it will be better than than the traditional methods. It just seems very complex considering what you are trying to do. One advantage of foam scenery is that it is very predictable - you can stack and carve away as much as you would like. Expanding foam, even from a mix like this is inherently the opposite - unpredictable. Even with a brush -on type its still going to potentially create unrealistic blobs that you will have to address which creates more work and more mess. I know that you are doing western scenery , but this stuff will tend to look like a fresh Hawaiian lava flow and may look "too smooth". Drop this stuff on the track (even with masking first) could create issues.  As you found out, this stuff can be very hard. I tried that in the past - Surform tools are very slow and difficult to use. The dust off this stuff is very fine and worse to some degree than pink foam bits. If you end up using a power tool to speed up the final shaping then you'll spray the dust mist everywhere which I get that you are specifically not wanting.

A quick relatively dust-free way is the old plaster gauze method - unroll, cut, dip, and place. You're not creating a hardshell, just a single layer to cover the foam seam lines/gaps/holes and it goes very fast. Larger gaps can be bridged with masking tape and then the gauze placed on top. There's many sources so you can by it in larger quantities than the WS stuff. Any soil materials/paint will cover the gauze and you're done.