Resin casting

We use "casting resin" all the time - we make sheets of different "textures" ie stonework, pantiles, etc, then cut to size for all our buildings and walling - and have had no real problem with mould wear. They will eventually show signs of degradation but it takes a lot of castings!!  

They're lighter than plaster, once washed take all paints well, are solid and relatively lightweight. Probably last longer on a portable/moveable layout considering plaster will break quite easily.

Having said that, plaster can give a better surface texture and will "soak up" paints and washes better. Depends on scale I suppose?  In smaller scales the surface may be too coarse unless you use a good quality casting plaster. We model HOe (HO-9 or HOn30 ) so want the better surface that resin gives.

Keith.

Thanks for the reply! As an

Thanks for the reply!

As an update, I made my first mold pretty successfully, but have not actually made a casting yet. After doing a lot of reading and seeing various opinions on how to go about it, I did not do anything to my plaster master, just poured the RTV over it. And it came out "OK" - I feel like I lost some of the fine detail pulling it out of the mold (fine grooves between sections) - I started with a simple concrete retaining wall section - I have a much more complex cut stone one to do too. Do you seal your plaster masters? I ordered some "paintable" mold release to try for both making the mold and making the castings. Some people say seal with acrylic spray, some say you don't need to do anything with plaster, but I have a feeling that cut stone master will not come out of the mold nicely if I don't do something.

I had to leave on a trip before I could try casting anything from my new mold, but I plan to have a go and report back when I get home next week.

-Kevin

Sealing masters

 As a norm, we give the master a very light spray with laquer ( gloss aerosol spray, auto repair spray is usually cheap enough over here in France)  But a lot depends on the quality of the plaster used. Proper modelling plaster is finer and gives a smoother textured surface, depends on what scale you model but builders plaster is only really good for builders!!

Mold release spray can be useful both for making the mold as well  as casting from it, although we don't bother with rtv silicone molds, they (the molds) take a lot of castings before they start to show signs of degradation.

Keith

Working time

In your instance, I'd be more worried about working time. Resin sets up quick, the stuff I use starts flashing at 90 seconds. You have to be quick on the mix, pour. I let my resin cure under pressure as well, so I have to factor that time as well.  

Plaster on the other hand has a very long working time. You can mix and let it sit for a long time . 

Since you are already having issues with RTV molds not picking up all the details ( I'd guess that it's due to air bubbles), I would suggest plaster. Your castings will only be as good as your master ( and the RTV mold). Say you get a 85-95% decent cast with resin. There's nothing you can do to fix it other than recast. With plaster, in theory, you can rewet and apply a light surface layer and replace details. This is just about impossible with resin.

If you want to use resin, just be prepared to have a steep learning curve and mentally prepare yourself for failures. When I started casting ( a similar basic wall section) I started at about 75% failure rate. Then slowly improved to 25% failure rate. Even today after many years of casting, I still have about a 5-8% failure rate. 

Another Option

Durham's Rock Hard Water Putty has been used to create castings.  This dry powder that you mix with water is far stronger than plaster and will take stains / paints.  Found at hardware / home improvement stores, the 16oz can sells for around three dollars, the 64oz can around eight dollars. 

Thanks for the further

Thanks for the further replies!

I did not have issues with the mold picking up details, nor are there air bubbles. The problem is that with the plaster unsealed, the RTV stuck to it a little. And thus I lost a bit of the fine detail, which was the fine lines between the sections of the "concrete" of the master (representing the forms into which the concrete was poured). Those very fine bits of the mold tore off with the master in places. Other than that, the mold looks great, even under a magnifier. Maybe I should have used a harder RTV?

It's still quite usable because I can always just scribe new lines into the castings. But I think this might be more of a problem with the more complex master for the cut stone wall Unless I'm overthinking it, nothing in the cut stone wall is as fine as those scribed lines in the concrete one. So for mold #2 I will try sealing the master. And I will try mold release too. My only concern, based on my research, is ending up with smoother, glossier castings than I want - this is supposed to be concrete or rock after all. 

I plan to just try both resin and plaster (I plan to use Hydrocal) and see what I like better, both in terms of making the castings and cutting and finishing the resulting wall sections. The resin I ordered has a relatively long working time. I fully expect to have bad ones, but the nice part of this is I don't need ALL of every casting anyway - they will all need to be cut down in height and one side or the other cut off to actually make the retaining walls anyway. Ultimately, I need about 3' of each type of wall, possibly more if I use it in other places on the layout. 

The Durham's suggestion is interesting, but I don't need these to be any stronger really - but I do need them to be easy to cut and finish. I actually already have some, might as well try it.

I'll report back with my results when I get home from my trip mid-week. 

plaster substitute

I have cast a lot of rock molds and was using Woodland Scenics.  That can get expensive and I looked for bulk hydrocal, but none of the building supply stores in this area had it.  I tried USG's DuraBond 90 and I like it.  It was fairly cheap and easy to find at Menard's.

Ken Vandevoort

Why even cast?

I’ve had good luck creating stone retaining walls with pink rigid insulation. I scribed tiers of stones by using a yardstick and the pointed end of a wooden skewer. Painted it grey, and weathered it with an India ink wash. It turned out great and people marvel that it looks so realistic. It’s a very quick and cheap way... no mixing anything!

Mostly because I want to know

Mostly because I want to know how to do it! There are a million things that casting is good for making. I certainly could have hand-carved the whole thing eventually, rather than just the masters.

Though hand-carving a cut stone wall is pretty darned tedious (doing the relatively tiny master was bad enough). Especially when it is better than 2' long and right at the forefront of the layout. The concrete walls are in a street underpass with four tracks going over it on bridges, so not quite as critical.  

-Kevin

I can get hydrocal at the

I can get hydrocal at the building supply places near me - but in WAAAAY too big of quantities for this project. In humid Florida it would go bad long before I would ever use it all, and I also have nowhere to store that much. A couple of pounds from Michael's will practically be a lifetime supply, even if it is much more expensive per pound (helped by their ubiquitous 25% off coupons).

-Kevin

Retaining walls

Kevin,

I did like you, I made my retaining walls from a mold that I made of a pre-purchased master. The difference is, I used latex rubber to create my mold. Then at one of the Fine Scale Shows I found a beautiful random stone wall silicone mold made by Sterling Models. I use it extensively now while my latex mold collects dust.

I’ve used Plaster of Paris instead of Hydrocal, I find it so much easier to work with. I’ve used it here as the retaining wall on the lead to my Barge Ramp. 

Ralph Renzetti

(fb)Weathering - A Touch of Yesterday

Plaster Substitute

Ken, 

I’m surprised that the builders supply did not recommend Plaster of Paris! It slightly softer than Hydrocal and yet harder than any of the the durabond plasters. You can also get it from a good art supply company, well you can here in Canada.
I got my first 20lb bag at Home Depot.

Ralph Renzetti

(fb) Weathering - A Touch of Yesterday

Plaster of Paris

Yeah, pretty much a standard casting plaster in the old days. It used to come in boxes about the size of Bisquick but I haven't bought any for years. I'm wondering how you plan to cut the hardened plaster retaining wall sections though? ......DaveB

US Gypsun No. 1 Casting Plaster

This stuff is engineered specifically for casting in molds. It had very good detail capacity and is pretty strong (not as strong as Hydrocal). I got 25lb bags of the stuff from a local pottery supply co. (Georgie's in Portland, OR).

I also use it for carving rock faces (I mix in some ballast for texture).

Charlie

Foam Retaining Walls

Expanding on Kenny D's comment, here are a few of my foam retaining walls, using 1/4-inch blue insulation foam. I freely admit that casting might bring better results!

 - Jeff

Early test:

Long curved wall using this technique:

Abutments on either end of a bridge:

foam retaining walls

  Hi Jeff,  I think those look fine. how did you hand scribe all the blocks?   I like that it would be easy to vary the size and shape of the stones or concrete blocks. The porous surface also looks like old pitted materials I've seen ....DaveB

I used a technique mentioned

Hi Dave -

I used a technique mentioned by Kenny D earlier in this thread.

Using a ruler and a fine-tipped marking pen, I marked horizontal lines on the foam. The distance between the lines defined the height of the blocks. Then, I used a thin metal straightedge to emboss the horizontal lines into the foam.

The next step was to use a flat-bladed screwdriver of the proper width to make the short vertical impressions to create the desired width of each block. I carefully measured the width of the blocks in the first row, making them all the same width. Then I used the first row as a guide to impress the rest of the blocks on the wall.

This went faster than I expected, since there was no measuring except for the first row.

The final step was to paint and weather the wall. I brushed on a light gray acrylic paint, then applied a fairly dilute India ink wash which seeped into the impressions created by the straightedge and screwdriver and also into various pits and defects in the foam, giving the wall some additional character!

It is a simple, low-cost technique that worked for me.

 - Jeff

Success! I made my first

Success!

I made my first casting tonight, from white resin. Came out great! I used a spritz of paintable mold release and it practically fell out of the mold. This stuff looks like it will be really easy to cut with a razor saw, or maybe even on my mini table saw..

I have a work trip to San Francisco this weekend (from SW FL), so no more until next week. 16hrs flying for 8hrs of work onsite, good times! Will make the mold for the cut stone wall when I get back, and try a plaster cast to see how that works out. Definitely like the speed of the resin casting, I could probably make all the wall sections I need of each kind in an hour or so. 1 minute of mixing, pour, then it's ready to demold in 15 mins. I can see where this will be more work to finish than plaster though - will need more than just India Ink washes.

Time for bed, 4:30 is going to come far too early - I live too far from the airport!

-Kevin

Ralph, that looks FANTASTIC!

Ralph, that looks FANTASTIC!

Jeff's carved foam looks good too.

Cutting hardened plaster

Dave,

Plaster of Paris is a softer form of Hydrocal. It cuts fairly easy but it will not accept excessive pressure with out breaking. I use one of three tools,

1) the back of a #11 blade applying just enough pressure to score your way through.

2) either a Hacksaw or Sabre blade mounted in a handle and used gently to saw through and square up with a block of sandpaper

3) a metal bastard file or rasp. I used the edge of the metal file to create grooves in the wall face to be able to inset the wood posts.

The colouring or painting is up to you, there are people that disagree with my painting methods, so it’s up to you!

Ralph Renzetti