Brush-On Low-Expansion Polyurethane Foam?

[C855B]

I've been shopping around trying to find a urethane foam or product of similar density that I could brush over cut pink foam as a workable finish coat. It would be instead of plaster or paper maché for scenery finishing. Most of the "over foam" finish coatings from the crafter and stage set designer markets are very hard shells such as epoxy, intended to give a truly finished, nearly polished surface for painting. "Smooth-On" is the most commonly available of these.

I guess I'm looking for a foam similar in density to Great Stuff, but low expansion (3X?) and brushable, not spray. The mix-it-yourself "pourable" foams from the moldmaking suppliers are 10X, and every demo video I've seen implies "Control? WHAT control?". IOW, you mix it in the target container, quickly stir, and stand the hell back.

Great Stuff Pro has a lot more control in the application, but it still is not something you're going to brush or squeegee around for an even finish. I'll go that way if necessary to fill voids and fix mistakes, but I'd like something that paints on with a longer cure time and some self-leveling properties. Does this exist?

[davefoxx]

I realize this is slightly non-responsive to what you're talking about, Mike, but I use lightweight spackling over my foam substrate.  Lightweight spackling doesn't require mixing and dries to a similar consistency of the foam.  So, you can easily sand or cut the spackling just as you can foam.  A plus is that you can apply the spackling in a thicker coat without the cracking that you get with joint compound after it dries.

Hope this helps,
DFF

[Sokramiketes]

If you spend the time on carving and smoothingthe foam, you don't need a skim coat of anything other than latex paint.

I once heard of a guy that mixed carpenters glue and expanded foam beads in place of other skim coats?

[Missaberoad]

Sculptamold isn't nearly as dusty as plaster or Joint compound.

I would still look into it as an option even if you're concerned about dust, I think the added level of control and better finished apearance would make it worth it.

The other possibility I see would be use lightweight spackling or Joint compound and smooth it with a wet sponge before it is completely dry. (obviously after it has set)

[C855B]

I already use lightweight spackling to level joints and rough spots. But it's far from "brushable". I find its dryness (versus, say, drywall mud) requires a lot of tool work, and that the pot always seems right on the verge of drying out during use.

As far as working the foam substrate "smooth" to begin with, no, that's not the process I'm after. I'd like to tool it into the basic form and then use [whatever] as a self-leveling coat, taking care of gaps, fillets and tool marks. The same purpose and result as Sculptamold or plaster, without the dust. Sculptamold may be less dusty, but the "dust" problem we're trying to prevent is long-term shedding, not what happens while working the material. Fiber products are going to shed even with attention to sealing with paint.

Here's a thought... has anybody tried thinning lightweight spackling? Is there a more liquid lightweight spackling, or is that just the nature of it? I think any common solvent would break down the binder.

For a while Lowes sold a high-build latex primer for such a purpose. It amounted to really thick paint, and would be close to the concept except for the workability and dissimilar density versus the foam.

[svedblen]

If you spend the time on carving and smoothingthe foam, you don't need a skim coat of anything other than latex paint.

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.

[SandyEggoJake]

@C855B

Mike, you might wish to look at www.alumilite.com products.

They make a line of liquid resins that expands like foam for a secondary filler in molds - used as a backer to reduce weight.  This one only expands about 3-5 x the liquid volume, and produces a skin suitable for sanding and painting.   

But you're right that you don't have much open time.  This one has a pot life of 45 sec (100g mass @ 75 degrees F) and expansion window of 75 to 90 sec. 

https://www.alumilite.com/store/p/972-AlumiFoam.aspx

However - and I'm TOTALLY spitballing here - as this is a two part 1:1 resin, perhaps ...just perhaps... you could brush (or spray) on one part on your layout and then brush (or spray) on the second part, mixing directly on your layout?  Of course, you'll want to test the resins separately and mixed on your pink foam to be sure it doesn't eat it up (too much).  And mixing on the layout is obviously problematic. 

-Jake

PS: I know next to nada about their products or the company, but try Lloyd Kluesner of Lloyd's Layouts who also reps for them and uses the products daily to produce his molded layout elements.  lloyd(at)lloydslayouts(dot) com 

[dmidkiff]

Hi Mike,

I've thinned lightweight spackle to pour roads and it improves the workability of the product greatly.  However, it also leads to the spackle shrinking and leaving rounded cracks once cured.  I sand the first coat after it had cured and then apply a second coat which fills the cracks of the first coat.  I thin with water and tint it with craft paint at the same time.

Doug

Here's a thought... has anybody tried thinning lightweight spackling? Is there a more liquid lightweight spackling, or is that just the nature of it? I think any common solvent would break down the binder.

[Scottl]

Check out Sculpture supply, lots of materials used for brush application.  They are Canadian but I am sure you can find the same products in the US.

sculpturesupply.com

[C855B]

Wow, guys! Great leads. Scott, it turns out that Sculpture Supply features the Smooth-On line but organizes it better than the maker's site, so I found "Foam-iT Rigid 10 Lb SLOW", with a pot time over three minutes, 6X expansion. That's going to be brushable. I hope I can find it in less than gallon quantities for testing.

Doug, I'm running a test right now having thinned the lightweight spackling with turpentine to a smooth brushable consistency. The hint to use turpentine was in the instructions that warned to wait 24 hours or more before painting with oil-based paints. This makes me wonder if the shrinkage problem you noted might be on account of the water, maybe?

[peteski]

@C855B

However - and I'm TOTALLY spitballing here - as this is a two part 1:1 resin, perhaps ...just perhaps... you could brush (or spray) on one part on your layout and then brush (or spray) on the second part, mixing directly on your layout?  Of course, you'll want to test the resins separately and mixed on your pink foam to be sure it doesn't eat it up (too much).  And mixing on the layout is obviously problematic. 

Wicked bad advice!  If the A/B parts aren't *THOROUGHLY* mixed, both parts will never fully cure, creating a huge mess.  And I just don't see thorough mixing being done by a brush and on a porous surface which already absorbed some of the unmixed liquid. 

[Hornwrecker]

I sounds like you are trying to recreate gesso with your thinned down spackling. 

[Mark W]

Wicked bad advice!  If the A/B parts aren't *THOROUGHLY* mixed, both parts will never fully cure, creating a huge mess.  And I just don't see thorough mixing being done by a brush and on a porous surface which already absorbed some of the unmixed liquid. 

The first thing I pictured reading the comment is this. (skip to 1:55 for the action)

Lightweight Spackle and latex.  No need to reinvent any wheels here.   I've even brushed on un-diluted Spackle before too.  Give it a try, it may just work. 

[SandyEggoJake]

Wicked bad advice!  If the A/B parts aren't *THOROUGHLY* mixed, both parts will never fully cure, creating a huge mess.

Oh, I think we all understand not well mixed and thus uncured resins can make for a sticky situation...literally. 

But to be clear, the application we are talking about isn't a "porous" substrate.  We are talking about on a closed cell foam like OC's FOAMULAR® 150 extruded polystyrene.   

And I'm NOT advising such in the slightest - especially if concept is to put down a THICK layer of each.  And pooling in pits would need to be avoided for this very reason.  The suggestion is ONLY an avenue for experimentation with resins with longer open time. 

Spraying a FINE layer and/or squeegee of A part resin, then coming back with  of the hardner  (NOT the other way around) MIGHT... again, MIGHT ... provide adequate mixing.  Again, we are not talking about nonexpanding resin either.  The exothermic reaction that get's produced will provide some additional mixing. 

On substrate mixing is done - and in fact recommended - in several applications.  For example, pouring a countertop over wood.  The common procedure is to first apply a THIN layer of unmixed resin on to the wood - yes even a porous substance like wood - to seal it, then poor the mixed on top.   And yes, some trades do spray mixed resins (though not something I'd let a novice do with my airless rig). 


@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. 

[Ed Kapuscinski]

I realize this is slightly non-responsive to what you're talking about, Mike, but I use lightweight spackling over my foam substrate.  Lightweight spackling doesn't require mixing and dries to a similar consistency of the foam.  So, you can easily sand or cut the spackling just as you can foam.  A plus is that you can apply the spackling in a thicker coat without the cracking that you get with joint compound after it dries.

Hope this helps,
DFF

This is what I've done, at Dave's recommendation, and I can highly recommend it.