Melted sprue in gap-filling.

[Cat_baloo]

Here I'd like to show what a little time and care can achieve in the battle against The Great Trench-digger, the generic problem of over-sized panel lines.

Over the years I've attacked this eyesore with varying success. Experience has shown me that filling lines with material matching closely the kit plastic is the most logical. Putty can be softer, and over-zealous sanding can leave a depression, Or brittle enough that dragging a scriber/pin/knife tip through it results in chipping that starts a vicious cycle and, at worst, ends with the model in uncontrolled flight into a hard surface.

Ask me how I know this.

 

One day I felt adventurous enough to use stretched styrene soaked in mineral turps/Turpenoid/white spirit, on my Airfix big Mustang. I'd snip off lengths to suit the line, coil up if necessary and sit it in a small glass jar (like Tamiya bottles) full of turps. The trick was not to leave it in too long otherwise it'd dissolve away to nothing. Basically, in an empirical study, I arrived at the formula – sprue has to be twice the width of the gap to fill.

As the sprue soaks in the turps the outer surface softens, then I mash it into the offending gap or line. The 'wet' sprue deforms well and spreads to fill the cavity, leaving enough material proud of the surface to allow for shrinking.

 

http://s4.photobucket.com/albums/y108/Riskyrossco/Airfix%20GB/?action=view&current=da879ec8.pbw

http://s4.photobucket.com/albums/y108/Riskyrossco/Airfix%20GB/?action=view&current=72943664.pbw

 

 

This is a very economical method, since you're only using precisely the length you need, to fill the line with no spill-over. The downside is s/times the sprue shrinks too much while drying and leaves dimples either side that require a little filler. However, if all goes to plan, these dimples are outside the line to be re-scribed, and there's nil danger of a scrappy scribing job.

A few pics of my 'Stang, complete with a final buff with auto polish.

 

Righty-ho, these are my weapons of mass extraction.

The ubiquitous Xacto, with much-resharpened No 11.

A splinter pick, rescued from some medical kit years ago. The hooked end is magic for carving fine lines.

A dental tool came out of a house pack, while I worked in an auction house years ago. In a moment of inspiration I took to it with a grinding wheel in a mototool to fashion a slightly hooked cutter.

 

 

On with the show. I'll be using my Revell F-14 for this exercise. The wing to show preliminary stages and the front fuz for later steps.

And here's the melted black sprue. I leave a tablespoon of turps over the goop, in storage, to prevent it drying too much. If it reaches the consistency of play dough, malleable between the fingers, it won't adhere. The slightly wet goop helps 'key' the plastic.

 

 

I tape either side, with 1mm or so of 'play'. As I toyed with variations on the theme I learned that the edges of panel lines are molded slightly raised and the rounded shoulder of the edges accentuates the width of the gap, if painted and weathered OOB . As we'll see the rescribed line has sharper edges and therefore appears thinner and more to scale.

I've smeared on enough goop to fill the length, with extra to form a domed profile. Since it doesn't stick to tape I can roll it back til it's mostly in and over the trench. To be safe I leave it three-four days to evapourate the residual turps.

 

Removal.

After two days the goop is firm enough to take a sharp, curved blade and start shaving off the excess sprue. It simply means less to file/sand later. The tape acts as a height guage, where I simply takethe excess sprue down to the level of the tape.

I can then remove the tape, for a better idea of how much sprue needs shaving.

 

 

Here's where the raised edge plays a part. It acts as an indicator. I use a fine needle file to gradually take the surface back almost to flush, finishing off the last material with 600-800 grit paper, and a quick polish with 1200-grit or higher. Otherwise there remains a slight 'dome' if I only use sandpaper.

I have used polish for a mirror shine, to aid scribing. But there's greater danger of the scriber 'skidding' off-line. Fine grit paper leaves a smooth enough surface for our purposes.

 

 

Scribing.

The most user-friendly straight edge has been my Tri-master templates. They're nicely flexible. Plus they have nifty gaps from the holes that are ideal for increasing adhesion area of the tape.

In a pinch I have also cut strips of printers' aluminium litho sheet, taped securely to the part I'm scribing.

My first pass is with the dental tool. I ground in a slight 'hook' at the very end, that carves out a very thin line instead of pushing the styrene out of the way. Using light pressure I pull it to slowly to form the guide line. Equally effective is the No 11 blade, held more vertical and pulled along back-of-the-blade first, to carve out the first material.

A light pass with fine grit paper, around a small balsa sanding 'block', smooths the burrs. The block prevents uneven sanding that can happen with fingers only. I slide the 'pin' tip of the pick tool along. At this stage the tip here is barely touching the bottom of the groove, to prevent digging in and skidding off.

 

 

 

To keep the new line clear of residue and swarf, between each step, I usually scrub it with an old 'fine' grade toothbrush dipped in soapy water. By now I might even give the plastic a quick swipe with polish compound, to check line depth. After polishing compound I always scrub with the brush.

 

 

Last thing is to give one pass with the curved tip, to clean up any uneven contours. By now the groove is deep enough that I can push the hook along, though still very light.

 

 

A quick pass with fine grit paper and scrub with brush and it's done.

I've swabbed a little Tamiya Sky on to aid the visuals. My new line does look quite wide. I've slightly exaggerated it for demonstration purposes; usually I'd take less material.

Still, compared to the black styrened gap I hadn't scribed you can see the difference in widths.

 

 

I tackled a complete re-scribe on a 1/72nd Hase P-3, after sanding off all raised lines, using similar scribing tools. That was a lesson on the durability of thick CA filler. But the panels lines were beautifully thin.

 

In conclusion, it's quite obvious this isn't an overnight, even two-day, exercise. The beauty, for me, is the use of kit styrene as filler. Nothing scribes like original plastic.

On top of that is also how easy it is for me to tackle a few lines, every now and then, between the obligatory halts in other, less intensive builds. Our motto in the Service was 'effective use of time and resources'. If you're doing one thing you can probably double up and fit in two. If in front of the idiot box it's no trouble to bring out the scissors, tape and bottle of sprue.

An hour or two can result in filling the gaps in a whole wing or fuselage, with a DVD playing.

Edited February 9, 2011 by Cat_baloo

[Out2gtcha]

Good stuff here Cat!

 

I currently use both plastic specific CA and 2 part auto filler, but as you pointed out, even the specific plastic CA can get really hard, especially if kicker is used, and the 2 part auto primer is great but dries VERY fast and can sometimes flake off if the panel lines are not too deep.

 

Thanks for the tips on the melted sprue. Always wanted to try this method, as I figured any gap filling material that was made from the exact same stuff as the surrounding kit material would be great way to go.

 

Cheers,

Brian

[ssculptor]

Thank you for a really good explanation of how to do this procedure.

Your photos and explanations are very clear and direct.

It is a very good presentation.

I like your idea of soaking the sprue in turpentine. I never tried that. I have used the liquid glue (solvent, really) but I'll try turps next time. You can always teach this old dog new tricks.

It may encourage some of the modelers to try to change the panel lines in their next build. But I suggest they start with a fuselage or wing from a partial kit from their scrap box. It is best to develop one's new skills on a junker than on Trumpeter's latest $175 kit.

Stephen

Edited February 9, 2011 by ssculptor

[MarkS]

Cat, what's the benefit of using Turps over a liquid glue? Turp is a very mild solvent and I would have thought that using liquid glue would provide better binding properties to the melted sprue. Just curious.

 

Excellent tutorial, BTW.

 

Mark S.

[Cat_baloo]

Cat, what's the benefit of using Turps over a liquid glue? Turp is a very mild solvent and I would have thought that using liquid glue would provide better binding properties to the melted sprue. Just curious.

 

Excellent tutorial, BTW.

 

Mark S.

 

 

Thanks for your feedback, Mark and Stephen.

 

I have Scots/English heritage. That means I "don't pay retail!" Turps is my favourite purely on price - it's only US$6 (or 3 Pounds) per litre.

I've tried automotive paint solvent, though its properties are different.

Liquid cement is Da Bomb. However, it's faster-acting and only penetrates a little, a lot less than turps. Basically it's designed to bond a strip of plastic to other plastic, rather than creating the appropriate 'sprue-goo'. With the right shape of stretched styrene it worked well but I did a lot of trial-and-error on the 'Stang to find the right 'stretch' to achieve the right width of sprue.

 

What it boils down to is I'm simply used to turps. It gives a workable medium, evaporates well and leaves the plastic hard. As an aside I'll throw some sprue into solvent and do some time-motion research.

 

Regards

 

Ross

Edited February 24, 2011 by Cat_baloo

[Out2gtcha]

Great explanation Ross, thanks.

 

On the subject of stretching the sprue, you mentioned above you stretch it and put it in the jar with the terps........

 

I was thinking of just taking a left over sprue tree from whatever build I was doing, and chopping some little sections off and placing them in a jar with some terps as apposed to stretching it in any way first.........would that method work in the fashion you've demonstrated?

 

Also, how long does it usually take for the sprue to turn to the "goo" stage after hitting the terps, and to that end, do you have to dump some terps out if the sprue becomes TOO liquidy?

 

sry to bombard you with so many questions Ross, but I find this technique very interesting.

Cheers,

Brian

Edited February 9, 2011 by Out2gtcha

[Cat_baloo]

Prot a noblem, Brian. All questions are relevant.

 

Pardon my confusing description. Making the sprue-goo only requires chopping up sprues into short lengths. No stretching needed.

If you want to avoid the rigmarole of using the gooey stuff THEN you can use stretched sprue dipped in turps or solvent - until the surface is soft but leaving a solid core, not completely melted. It's tidier and economical yet can shrink more, only because of the simple reason there's less sprue volume.

 

Here's how I do it, on an F-14 wing I used some time ago for a similar, albeit abridged, 'show-and-tell'.

 

Once it's soaked enough I place it in the gap, holding one end with tweezers and guiding in on with a knife tip.

 

 

A knife handle is great for rolling it flat. Makes it easier to sand down since it's already near flush.

 

 

The salient difference between turps and solvent is how the sprue melts.

Turps turns the sprue into a glutinous mass BUT the excess liquid remains separate and floats above it, and is able to be poured off leaving the molten plastic in a liquid putty consistency. Whatever kit plastic is used the goo reaches the same viscosity.

 

Solvent combines with and completely dissolves the plastic, producing a 'soupy' sludge. To produce the equivalent 'putty' consistency requires stricter ratio control of solvent to plastic.

Either I start with a little solvent and gradually add to achieve the correct viscosity, or cover the sprue chunks, wait til it's fully molten and add sprue to adjust. Since it's 'hotter' it creates a sludge in under 30 mins.

 

Having said that, the applications of solvent-based goo are promising. F'rinstace, it's runny and can be used to smear over slight blemishes, such as scribing mistakes or ejector pin marks. with little collateral damage. When I used CA gel to fill scars I would put a small patch of clear (Sellotape) tape over the spot and that would serve to flatten the CA.

Made it far easier to file/sand of the excess w/out taking off too much surrounding plastic. It's all in the preparation - 10% forethought eliminates 90% of the work!

 

Edited to add: Dipping time for stretched sprue is between 30 sces to a minute, for 0.5mm to 1.5mm widths. A clear indicator is the sprue starts to look 'furry' and lightens in colour.

 

Hope it helps.

 

Regards

 

Ross

Edited February 9, 2011 by Cat_baloo

[LSP_Kevin]

I was thinking of just taking a left over sprue tree from whatever build I was doing, and chopping some little sections off and placing them in a jar with some terps as apposed to stretching it in any way first

 

Brian, I've done exactly that in the past using lacquer thinner instead of turps. It was an interesting experiment, to be sure. The main problems I encountered were that the resultant goo tended to shrink while drying, tended to develop trapped air bubbles if applied too thickly, and had a certain brittle quality the original plastic lacked.

 

I'd be interested in hearing from Ross whether using turps solves these problems. Or maybe I'll just try it myself!

 

Kev

[Cat_baloo]

Valid point, Kevin.

Most of the preliminary experiments were to investigate how far I could go for width of stretched sprue to counter this effect.

If the sprue 'wire' was too large it wouldn't penetrate the depth of the trench. Thus the move to melted sprue.

I had no probs with air bubbles. If there were any they tended to dissipate as the turps evaporated. The jury's still out in re: use of solvent. But I'll remedy that in next post.

 

Regards

 

Ross

 

Edited to add: Regarding brittleness, turps has nil effect on radical change to the molecular characterisitcs. Styrene is a thermo-forming polymer and seems to take multilple melting without undue change.

If anything, I found it does become 'grabby' on the sandpaper if not fully set. It still sands out cleanly but there's a slight squeak if I sand quickly. Taking it slower eliminates the grab, and giving it another day to dry helps.

Edited February 24, 2011 by Cat_baloo

[Out2gtcha]

Awesome Ross, thanks.

That answered my main question, which was basically just asking (in a round about way i guess) whether or not the melted sprue "mixes" with the terps like i found it does with solvent if you let if completely dissolve. Thats what confused me a bit when you mentioned that you put a tablespoon of turps over the goop, in storage.\

 

Nice to know the terps remains separate from the goo and can be pored off.

 

Maybe I missed it in the above description, but how long does it take the cut up sprue pieces to become goo?

 

Brian

[Cat_baloo]

Awesome Ross, thanks.

That answered my main question, which was basically just asking (in a round about way i guess) whether or not the melted sprue "mixes" with the terps like i found it does with solvent if you let if completely dissolve. Thats what confused me a bit when you mentioned that you put a tablespoon of turps over the goop, in storage.\

 

Nice to know the terps remains separate from the goo and can be pored off.

 

Maybe I missed it in the above description, but how long does it take the cut up sprue pieces to become goo?

 

Brian

 

 

Brian, I usually give it a full day, more like two to be safe, for the turps to act. Fun thing is the styrene only absorbs what it needs to melt and no more. Hence the excess left floating above. Really funky to compare, and I wish I'd stuck with kemistry in skoole!

Solvent, as above, takes less than an hour. As it's ALL solvent it simply eats up sprue as long you feed it. Until it reaches saturation level. Sorta the reverse of turps, if you like.

Must say, having to explain, to look from the outside in, so to speak, I'm only starting to really understand this whole thing systematically.

 

Regards

 

Ross

[thierry laurent]

Funny!

 

I discovered by accident this property of turpentin thirty years ago when I was a quite young modeller.

 

I used to leave my brushes in a turpentine jar to remove Humbroll paint residue. I received some cheap brushes with a plastic handle and did what I used to... to discover some days later that the brushes had disappeared! Guess what? The jar was full of a dirty green-brown goo!

 

However, I've never considered re-using this to fight against the trench syndrom!

[Cat_baloo]

Ain't that the funny part, Thierry? much of my 'technique', if you can call it that, stems from happy accidents.

 

Put in a few days experimenting with solvent-based goo. Really the quicker and easier, simply because it's so much faster.

Here's a comparison between turps on the left, solvent on the right (with ESCI F-4E kit plastic in case you're wondering).

 

 

Applying it needs more care, since it likes leaving 'string' everywhere. Pays to twirl the applicator (my dental tool in this case) to ravel up any excess. Sets with no pin holes or cavities, too.

 

 

It can dry in 24-36 hr, depending on the thickness of the dose. I still leave it at least 48hr before filing or sanding, to be safe. But I carve away the bulk after less than a day.

If thinned out a little, more like a runny adhesive (which it basically replicates), it can be brushed on.

 

All's said and done the only advantage with solvent is the slightly quicker set time.

 

Happy plasticking.

 

Ross

[Shawn M]

this is great information, thank you!

[LSP_Kevin]

Well, as it happens, I've had the opportunity to experiment with the sprue+turps method sooner than I'd anticipated. The engine nacelles on my 262 are giving me hell at the moment (mostly my fault, since I glued the wrong halves together ). After filling all the resultant gaps with various combinations of styrene, Milliput and CA, I set about rescribing the lost panel lines. In some spots I managed to gouge out great lumps of filler, and it looks terrible. I've refilled these areas with the sprue goo, and will see how it turns out after sanding and rescribing.

 

Kev