Model Train Forum banner

1 - 11 of 11 Posts

·
Registered
Joined
·
2,170 Posts
Discussion Starter #1
I'm making a shelf layout which I want to keep fairly lightweight, so I was wondering if there are any techniques for using regular spray foam with Woodland Scenics rock molds? Since this kind of foam is a chemical reaction I'm worried about it releasing from the mold.

If this works, what I have in mind is to paint the foam 'rocks' with a brush-on latex paint to solidify the surface, use a knife to deepen any features that get lost in the paint (or to add more detail), then I can start painting and washing them like normal.
 

·
Registered
Joined
·
94 Posts
I wouldn't use a direct spray foam on the rock molds, it won't release properly .. instead you -might- try a wash of plaster or similar inside the rock mold first, then finish it with spray foam ..
 

·
Registered
Joined
·
3,135 Posts
Or spray silicone on the inside of the rock mold before using the spray foam.

The problem with using that as a release agent is that painting will be difficult.
 

·
Registered
Joined
·
1,172 Posts
I think it would stick, the great stuff foam is very sticky. You can try a layer of something between the foam and the mold... But I think I'd prepare for failure. I thought latex myself...
 

·
Registered
Joined
·
4,110 Posts
I’m thinking that using polyurethane spray foam in a rock mold would result in losing the mold. Lightweight hydrocal is somewhat lightweight and most rock molds are fairly thin, so I don’t think hydrocal rock formations would add a whole lot of weight.

There is one rock mold (C1236) that takes a lot of hydrocal. I have stuck scrap pieces of rigid foam in that mold before the hydrocal cured. I did it to stretch my hydrocal supply, but it also made the rock a bit lighter.

Some other options if you want to keep things light is use rigid foam as the main base and gluing hydrocal rock outcroppings or use rigid foam and the carve rock into it. Here is a shot with both techniques.

542103
 

·
Registered
Joined
·
1,172 Posts
Well I just bought 3 molds and a carton of hydrocal myself. Haven't made anything yet. But I have pink foam with some aluminum screen formed over it. I'm not quite done with the form. I use gorilla glue to hold it all together. As for the rocks. I haven't made any but was just thinking I'd glue it to the screen directly, possibly with something like paper towels glued over it first. I've done that step in another section and it worked for me. So I figured adding rocks would be ok. Having said that I don't have a large rock area. But... Perhaps one could take some long deck screws, drill those through the various pieces into the backing with some glue... And that'd do it. Assuming the backing is beefy. As to your efforts -- they ook great. I like the idea of hollowing out the rocks or using some other material also. A long long time ago I had a plaster mold ... We used Vaseline in it to make the cast. Kinda messy but it did prevent it all from sticking together. Lost some details. The suggestion of using silicon or say Pam cooking spray or similar could work in the doems... But it might not. If I had some mold I didn't like I'd be tempted to try it.
 

·
Registered
Joined
·
1,172 Posts
Here's something I dug up with mr google. it's a hot glue take on it. Looks decent for ice ... not sure they give enough details on how they prevent sticking. I did find a reference to using foam from 2001 -- although sticking to the mold wasn't mentioned, detail and gap filling issue were. What's old is new and what's new is old, I guess.

 

·
Registered
Joined
·
899 Posts
If you have a large rock face, you should look into using Bragdon Enterprise's (Bragdon Enterprises - Cutting Edge Model Train Scenery Technologies) rock molds and resin products. I used several of their molds on my layout. They use a plastic resin to make the surface of the rock face and then use expanding foam behind it for support. It worked very well for me and it is extremely light weight. If you want to see what it looks like on my layout, check out post #15 and #17 in my layout thread (Modified Peace River HO Layout). Another advantage of this method is that it is also very durable. It won't chip or crack like plaster or hydrocal can.
 

·
Registered
Joined
·
2,170 Posts
Discussion Starter #9
Sorry for disappearing from this thread! Never saw a note that there had been replies.

So to clarify some questions, I was looking at smaller rock formations for this. The larger stuff would just be piled up foam, glued and scraped for shaping, but these smaller rock molds have some nice detail that would have been nice to include. Plus if this had worked, I could have stuck the molded rocks into the wet spray-foam on the larger structures and scrapped the edges to completely blend everything together.

I know spray foam is a pretty nasty chemical reaction, it gets on things and never releases, so you just never know. Powdering the mold won't work, and I'm not even sure I would trust the plaster wash.

Hmm, maybe something lightweight like plastic wrap, spray a bit of foam inside then flip it right-side-up onto a board and weigh down the sides of the mold so as the foam expands it pushes the plastis wrap up into the crevices of the mold? I dunno, the stuff is cheap, maybe I'll do some experimenting and see if I can make it work while retaining the details of the mold.
 

·
Registered
Joined
·
1,172 Posts
Maybe PAM cooking spray? We used vasoline in plaster casts way back. Kinda worked but lost detail.

I just coincidentally poured plaster into some molds I bought weekend before last.

No results but I'm not sure about it somehow. One mold got a very wet mix and the other got a somewhat smoother mix. I measured the same way both times!

In my local hs they have a bunch of rock formations already done in something like latex. Very tempting!
 

·
Registered
Joined
·
1,172 Posts
Ok quick update. I waited the prescribed 40 mins or so and popped it all out. And I'm going to say it worked! Some cracked but with a little practice and I think thin lube between the plaster n cast (they recommend 'wet water' which is just water and a little soap... ). It seems it should/could work with other materials. Still you'd want with foam something it really won't stick to...
 
1 - 11 of 11 Posts
Top