Sprue Goo

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Guy Rixon
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Joined: Sun Mar 27, 2011 6:40 pm

Sprue Goo

Postby Guy Rixon » Sun Feb 11, 2024 12:30 pm

"Sprue Goo" is a filler for polystyrene made by dissolving plastic scrap in solvent. I came across it in the works of Youtuber James on channel LPJ models, but I never expected to use any. I have in stock Tamiya fine filler which serves the same need.

Then I came to this.

It's a roof for a PLV made as a box section where the curved part is styrene "planks" laid over styrene ribs, the plank gaps filled and the whole sanded smooth.

I filled the gaps with Tamiya filler and all was well, until I distorted it slightly when drilling for the roof furniture. The planks bent, the filler pinged out and I had to pry at it with a blade to pull the planks back into alignment. Clearly the filler needed to add structural strength as well as cosmetic goodness.

Therefore sprue goo. It has at least a chance of welding the plastic into a single, strong section. You can't buy it AFAIK, so I made some up.

It's normally made from kit cast-offs and is greyish, but I wanted white, so I cut up some white styrene off cuts that have been at the bottom of the scrap box for a good few years. I used an empty plastic-solvent bottle to mix it as the bottle was a suitable size and came with a brush for putting the goo onto the model. The solvent was Humbrol Liquid Poly, which is an aggressive solvent, good for total dissolution and less good for delicate assembly. The best mix of plastic to solvent is unknown to me. I used scrap cut into roughly 5mm x 5mm x 2mm lumps (with snips) and then piled up in the bottle to the surface of the solvent. I left it four hours and found it had gone to a decent consistency, so put it on the roof.

Today, about 20 hours later, it's set nicely, the roof hasn't melted into a blob, and it feels stronger than before. At the least, I managed to sand the goo back without breaking the joints. The picture, incidentally, is the roof after goo-filling and sanding. It needs a closer look and more sanding to finish, but it's getting there.

David Thorpe
Posts: 46
Joined: Mon Jan 08, 2024 10:01 am

Re: Sprue Goo

Postby David Thorpe » Sun Feb 11, 2024 1:02 pm

Useful. I assume that MEK, of which I have a large bottle, will have the same effect so I'll give it a try as I currently have quite a lot of spru cast-offs.

martin goodall
Posts: 1424
Joined: Fri Oct 02, 2009 6:20 pm

Re: Sprue Goo

Postby martin goodall » Sun Feb 11, 2024 8:26 pm

My recollection is that George Slater himself invented "sprue goo" at least 60 years ago, and he used to demonstrate its use on his trade stand at the MRC's Easter Show at Central Hall, Westminster (and no doubt at other shows).

Styrene sheet (or 'Plastikard' to give it the trade name that George Slater adopted) has been a favourite constructional material for me ever since, although I don't often resort to making sprue goo nowadays.

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Joined: Sat Jun 03, 2017 1:09 pm

Re: Sprue Goo

Postby Daddyman » Mon Feb 12, 2024 7:18 am

Guy Rixon wrote: I managed to sand the goo back without breaking the joints.

While there might be uses for sprue goo, it doesn't seem like it's bringing much to the party here. Watered-down Milliput can be smeared into joints such as these so that no sanding is required.

Guy Rixon wrote: styrene "planks" laid over styrene ribs.

My concern with this method was that it wouldn't be strong enough, and your experience seems to bear out my fears. Rather than use ribs, I made the former from "slabs" of 40 thou which go the full length of the carriage between the ends; full-strength Milliput is used to take the "steps" out of the former, before the planks are laid on top. You could use the resulting roof as a cricket bat.

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