One thing I haven't seen mentioned about the extruded foam (eg Celotex/Kingspan stuff) is that it can shrink like heck after cutting.
I used it for the scenery on Coniston and one length I cut and carved to make the cliff behind the goods yard went like a banana surprisingly quickly because the underside was the original surface as manufactured and the upper side was carved.
Therefore it's important to seal any cut surfaces as soon as you can - you don't want baseboard edges shrinking away from their framing or from each other.

Technically Celotex is not any form of Polystyrene, its polyisocyanurate Foam with a sheet of tin foil top and bottom..

If you use Celotex, the strength is in the tin foil, if you remove one side it will turn into a banana. You have to use a complete sheet WITHOUT carving as a baseboard, Glue another on top and carve that.

I've not had any problem with shrinkage because once a board is cut to size, then edging is imediately glued on, including as a single piece that for the edges of any upper layers..

Quote from: chrism on September 13, 2019, 06:31:35 AM
One thing I haven't seen mentioned about the extruded foam (eg Celotex/Kingspan stuff) is that it can shrink like heck after cutting.
I used it for the scenery on Coniston and one length I cut and carved to make the cliff behind the goods yard went like a banana surprisingly quickly because the underside was the original surface as manufactured and the upper side was carved.
Therefore it's important to seal any cut surfaces as soon as you can - you don't want baseboard edges shrinking away from their framing or from each other.

What would you use to seal the edges? Should this be done after I split the boards (and before I add the ply frame?)

Quote from: Thebaz on September 13, 2019, 02:14:58 PM

Quote from: chrism on September 13, 2019, 06:31:35 AM
One thing I haven't seen mentioned about the extruded foam (eg Celotex/Kingspan stuff) is that it can shrink like heck after cutting.
I used it for the scenery on Coniston and one length I cut and carved to make the cliff behind the goods yard went like a banana surprisingly quickly because the underside was the original surface as manufactured and the upper side was carved.
Therefore it's important to seal any cut surfaces as soon as you can - you don't want baseboard edges shrinking away from their framing or from each other.

What would you use to seal the edges? Should this be done after I split the boards (and before I add the ply frame?)

A liberal coating of PVA seemed to fix mine OK. Obviously, being just scenery, it soon got paint, etc. on top as well.
I'd suggest you do it as soon as possible after cutting.

Quote from: Thebaz on September 13, 2019, 02:14:58 PM

Quote from: chrism on September 13, 2019, 06:31:35 AM
One thing I haven't seen mentioned about the extruded foam (eg Celotex/Kingspan stuff) is that it can shrink like heck after cutting.
I used it for the scenery on Coniston and one length I cut and carved to make the cliff behind the goods yard went like a banana surprisingly quickly because the underside was the original surface as manufactured and the upper side was carved.
Therefore it's important to seal any cut surfaces as soon as you can - you don't want baseboard edges shrinking away from their framing or from each other.

What would you use to seal the edges? Should this be done after I split the boards (and before I add the ply frame?)

If you are putting the frame on then the liberal coat of PVA can be used to glue the frame on at the same time..

Well, I've been doing my homework and I've settled upon a foam baseboard design which incorporates many of your suggestions from the dowels to the toggle catch. Utilised my AutoCAD skills to knock up a plan that I'm happy with and will be able to take off the quantities to make sure I'm buying the right amount number of bits (shouldn't be too hard tbh!!)

See pdf attachment for plans and let  me know if you have any comments 

I note you've marked up for expanded polystyrene,  I would recommend extruded polystyrene it's much stiffer and better for baseboards..

Yes you absolutely need to use XPS, Extruded Polystyrene. Here in the US its used as wall insulation and is available 1/2", 1", and 2" thick. It is much denser than white polystyrene foam used as packing material, so it can be cut much more accurately, without a lot of mess.

Regards,

John P

Quote from: The Q on September 24, 2019, 08:28:59 PM
I note you've marked up for expanded polystyrene,  I would recommend extruded polystyrene it's much stiffer and better for baseboards..

Quote from: jpendle on September 25, 2019, 01:33:58 AM
Yes you absolutely need to use XPS, Extruded Polystyrene. Here in the US its used as wall insulation and is available 1/2", 1", and 2" thick. It is much denser than white polystyrene foam used as packing material, so it can be cut much more accurately, without a lot of mess.

Thanks for looking at my plans and commenting. Truth is I haven't yet decided whether to use polystyrene or say, the Kingspan stuff. I've read of people making a success out of using polystyrene, so I wanted to keep that as an option especially as the XPS stuff is about 400% more expensive! I note your points about XPS being a lot easier and therefoere, neater to cut so I suspect I will probably go down that road in the end.

My main concern now is cutting the ply as I only have a manual carpenters' saw. Should I try to get hold of better cutting equipment or would I do fine with the manual saw. I've read about the correct side up to place it for cutting so presumably if I stick to that I should be okay?

Quote from: Thebaz on September 25, 2019, 09:04:41 AM

My main concern now is cutting the ply as I only have a manual carpenters' saw. Should I try to get hold of better cutting equipment or would I do fine with the manual saw.

Although I have electric circular saws, jig saws, and a band saw, I still prefer to cut ply baseboard frame pieces with a hand saw on a Workmate. Just make sure the teeth are in good condition and sharp. Cut with the teeth running on the outside edge of the cut line, go gently without lots of pressure and let the saw do the work.

Quote
I've read about the correct side up to place it for cutting so presumably if I stick to that I should be okay?

Don't think I've ever heard of that in all the decades I've been cutting ply. Presume it's maybe something to do with grain direction when cutting?  I've never had a problem.

I have cut  thick (9mm) ply with a jigsaw all me life without problem.

And that includes a circular hole for a Fleischmann  turntable!

However, eye protection is recommended. 

Quote from: ntpntpntp on September 25, 2019, 10:15:09 AM

Quote from: Thebaz on September 25, 2019, 09:04:41 AM
I've read about the correct side up to place it for cutting so presumably if I stick to that I should be okay?

Don't think I've ever heard of that in all the decades I've been cutting ply. Presume it's maybe something to do with grain direction when cutting?  I've never had a problem.

Since most ply is an odd number of layers, the grain will run the same way on both surfaces so I doubt it's that. Ive never had a problem either, cutting in whichever direction I wish.

If it is warped, I cut from the up side, while trying to keep the ends level.

I do this on a driveway, which itself is on a slope! 

I think that have a clear game plan, and take care is the solution here.

Some Plywoods are "Good One Side" that means the top layer one side should have no blemishes or patches. The really cheap DIY shed stuff is often of this type.
If you are using it varnished in furniture you need to cut and use the correct side visible.
For a model railway you're going to cover it in paint and polyfilla anyway so for a flat earth top it doesn't matter. If you are however going to use it for say a lighting arch facia. Then get the correct side facing out, you don't want the patches showing through the paint.

As for what to use, a electric jigsaw with a comparitively smooth blade is good for cutting precision smooth edges. But Chippendale used hand tools, you can produce quality work with hand tools, it just takes longer.