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I've got my track 'adequately' laid. Its largely a unitrack track on pink SM type of extruded foam. Little of it is flat, most are inclines, even slight ones.

At the moment I've used silicone caulking to hold the track in position. In some places it is up to 3/4" thick. I have a bunch of tracks parallel to each other (basically double tracked). I have a gap down to the foam in between the two sets of rails. I'm debating as to what is the best method to fill that up. I initially planned to use the expanded foam, and will where the desired thickness is greater. What would you suggest for these spaces? The foam expands and can make a righteous mess, so I was wondering if plaster of paris would stick, or if there is a better idea.

Thanks
 

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Expanding foam no!

I've got my track 'adequately' laid. Its largely a unitrack track on pink SM type of extruded foam. Little of it is flat, most are inclines, even slight ones.

At the moment I've used silicone caulking to hold the track in position. In some places it is up to 3/4" thick. I have a bunch of tracks parallel to each other (basically double tracked). I have a gap down to the foam in between the two sets of rails. I'm debating as to what is the best method to fill that up. I initially planned to use the expanded foam, and will where the desired thickness is greater. What would you suggest for these spaces? The foam expands and can make a righteous mess, so I was wondering if plaster of paris would stick, or if there is a better idea.

Thanks
DavidJones;

I do not recommend using expanding foam at all. It is like an uncontrolled puffed up spray of super glue and does indeed make a serious mess! Like its liquid cousin, super glue, It bonds to human skin so firmly that it sometimes can only be removed by sanding it off! Also the amount of expansion is uncontrollable. It tends to get onto, and into, places where you don't want it.
If you decide to use this stuff then at least protect yourself with safety goggles, disposable rubber gloves and an old, potentially disposable, long sleeved shirt.

Silicone calk 3/4" thick sounds like an awful lot of caulk! Normally a thin layer 1/32" or so is sufficient to glue the track down. In the case of roadbed track, like the Kato unitrack you are using, a bead of 1/16" or less thickness along both bottom edges of the roadbed piece should be plenty to hold the track in place. If you're using calk as a filler/sub-roadbed, I suggest using wood instead as it is, less flexible, and therefore a lot more stable, than a thick pile of caulk.

As for filling gaps between pieces of extrude foam board; Plaster might work temporarily but it might also crack later, especially if it's thin. Plaster of Paris is known for its fine detail molding capability and fast drying, not for exceptional strength. Hydrocal or Ultracal, from U.S.Gypsum Co. are much stronger plasters.
Ideally there shouldn't be any serious gaps between sheets of extruded foam, they should butt up against each other pretty tightly. But we don't live in an ideal world, and there will be some gaps, hopefully not to big.
A Low-temperature hot glue gun does a good job of bonding pieces of extruded foam and can also fill some small gaps. caulk can also be used to fill gaps, that after all, is one of the jobs it is designed to do.
I also use Elmer's wood filler as a gap filler and general scenery material instead of plaster. That's just my personal preference, plaster will do the same job.
Plaster-impregnated cloth, like that sold by Woodland Scenics, is another possibility. It can cover any wide gaps. Wood strips could be cut and sanded to shape, and glued into any gaps where extra strength, or a flat surface, is needed.
So, you have many choices available.

Good luck Have fun!

Traction Fan :smilie_daumenpos:
 

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Thank you - that was very helpful

I would prefer to have thinner silicone. What inevitably happens is I need to shift something just a couple of mm higher, often at one end to make a smooth curve (and keep the GD thing running). I'd say often it goes on in layers- I'd have it maybe 1/4" up, but then discover the locomotive is swooping upwards, so I'll lever it off with a putty knife, put some more silicone goop underneath, press it down until it looks 'right'. The stuff seems to be fairly good. Not as solid as wood, but as solid as the foam. It might move a bit, but it takes a fair force to do so.

I like the hydrocal idea. I like the getting rid of the expanding foam idea. I agree- I need to let it, and superglue age off my skin. ie it stays on until the skin falls off. All my best laid plans were used to start everything :)D), then, as the gastroenterologist say, 'crap happens'. I am a bit OCD in my woodworking hobbies and generally won't leave something until its adequately perfect, so there is lots of under the surface fiddling that goes on before it comes out right. And often just replacing things.

I build (built) boats and am very familiar with using fibreglass/carbon fibre/kevlar cloth and resins. I am also a physician and have made my fair share of casts. I am thinking of marrying the two and trying to find an open weave cloth that I could lay on top of the hydrocal, then cover with a touch more (like putting fibreglass on a boat, then covering it with resin). It would add some tensile strength so the plaster doesn't crack as easily/often.

I was going to rebuild some of the mountains I trimmed down to make it work with the expanding foam. It can be worked similarly to the SM pink insulation (I think, but am not sure that it is chemically the same product). It has a much coarser texture when carved. And can make a righteous mess. I will cut new SM insulation instead, then use some of these methods to close the little gaps left- likely the glue would work best as, due to my OCD woodworking tendencies, I'm likely to have gaps under 2mm

Thanks for your help!!!!
 

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What is "SM" foam?

Thank you - that was very helpful

I would prefer to have thinner silicone. What inevitably happens is I need to shift something just a couple of mm higher, often at one end to make a smooth curve (and keep the GD thing running). I'd say often it goes on in layers- I'd have it maybe 1/4" up, but then discover the locomotive is swooping upwards, so I'll lever it off with a putty knife, put some more silicone goop underneath, press it down until it looks 'right'. The stuff seems to be fairly good. Not as solid as wood, but as solid as the foam. It might move a bit, but it takes a fair force to do so.

I like the hydrocal idea. I like the getting rid of the expanding foam idea. I agree- I need to let it, and superglue age off my skin. ie it stays on until the skin falls off. All my best laid plans were used to start everything :)D), then, as the gastroenterologist say, 'crap happens'. I am a bit OCD in my woodworking hobbies and generally won't leave something until its adequately perfect, so there is lots of under the surface fiddling that goes on before it comes out right. And often just replacing things.

I build (built) boats and am very familiar with using fibreglass/carbon fibre/kevlar cloth and resins. I am also a physician and have made my fair share of casts. I am thinking of marrying the two and trying to find an open weave cloth that I could lay on top of the hydrocal, then cover with a touch more (like putting fibreglass on a boat, then covering it with resin). It would add some tensile strength so the plaster doesn't crack as easily/often.

I was going to rebuild some of the mountains I trimmed down to make it work with the expanding foam. It can be worked similarly to the SM pink insulation (I think, but am not sure that it is chemically the same product). It has a much coarser texture when carved. And can make a righteous mess. I will cut new SM insulation instead, then use some of these methods to close the little gaps left- likely the glue would work best as, due to my OCD woodworking tendencies, I'm likely to have gaps under 2mm

Thanks for your help!!!!
DavidJones;

What is "SM" pink insulation foam? Is it the same as extruded foam insulation board sold by Owens Corning wich is pink? If so, you can lessen the mess considerably by cutting it with either a hot wire foam cutter tool, ( www.micromark.com ) or an electric hot knife tool, ( www.harborfreight.com ) Either will melt through foam without producing any dust, and leave a clean edge. They do produce some nasty fumes though, so have plenty of air circulation when using either tool.
For hills, mountains, etc. just stack layers of pink foam like the layers of a cake. They can be glued together with white glue, a low temperature hot glue gun, or Liquid Nails For Projects (make sure the container says "for projects". Regular Liquid Nails is solvent-based and will melt foam. Then carve them to shape. This works better than trying to form scenery from spray can expanding foam, and it's safer, in that nothing sticks to your skin. The hot tools can do the initial rough shaping.You can use a surform rasp (see photo) from Home Depot, for intermediate shaping. I hold the nozzle of a shop vac right next to the rasp to catch all the dust it generates, before the dust gets all over the place. Final smoothing of the terrain can be done with sandpaper.

Have fun,

Traction Fan :smilie_daumenpos:

Surform rasps.jpg
 

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Anything over 1/4 , and especially 3/4 is awefully a lot to fill. I would suggest building the level up with scrap pieces of the pink foam. If it comes up higher the needed its fine too, can be shaped with retractable knife or the tool pictured above.

For filling I wouldn't use plaster of Paris, to heavy, perhaps only if mixed with horticulture grade vermiculite. Lightweight spackling can be used .

Expanding foam works great actually if used carefully. I do not disagree on points brought up above , but it is perfectly if one plans and accounts for it's properties. expansion indeed can be unpredicted, but if things are masked the excess can be cleanly cut off layer.

Good luck!
 
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