Expandable Foam?

[LIRR]

I've used it, it's great for filling odd shaped gaps, but I noticed it doesn't take paint.....

[fotoflojoe]

You should check out these sabre saw blades for cutting foam.  They changed my life.   Some examples starting here.

This!
Also, if you're making long, straight cuts,I find that running foam sheet through the table saw - just like plywood - works very well.
Straightedge guide or not, I'm really bad at cutting foam board using the score-and-snap method.

-Joe

[SandyEggoJake]

+1 on the tablesaw... with a good thin kerf blade to keep the dust to a min. 

This is the type the pro's use:
https://www.amazon.com/Bullet-Tools-CenterFire-Poly-ISO-insulation/dp/B00KM0RLUU

[MK]

That's cool but at $50.  I guess you have to pay to play.    Be sure to watch the video on Amazon.  Looks like you can cut foam indoors now!

[davefoxx]

Thank you guys, it's probably any where from 1/4" to 1 1/2" of a gap.

Scrap foam pieces and lightweight spackling is my choice to fill in voids on my layout.

DFF

[sizemore]

I would probably opt for Great Stuff (foam in a can), as long as the area has an open side the foam will expand in the direction of the opening. Allow it to set overnight. Then you can use a sharp serrated blade and rasp to form the foam when dry.

If you use a two part expanding foam that requires a chemistry degree to mix, then you might run into issues with expansion and warping things.

The S.

[Boilerman]

I used great stuff on my layout to build slopes and fill gaps.
You need to let it set for a day prior to carving it and then it will expand some more needing to be carved again.
Once I have the desired shape I put 2 layers of plaster gauze over it.