Craig

I’m liking these Friday posts which I’m thinking more as commentary, or a research opportunity. I like writing, so these give me a chance to write even if I have nothing in particular to report about my hobby.

I thought I’d follow-up on my post from last week about the use of cyanoacrylate as a gap filling solution.

Companies have certainly made huge strides in making sure things fit together well. I remember assembling my Ork Battlewagon, with it’s amazing fitting panels, and believing it to be the work of the divine. The Jaws of the Deep models I assembled last month were pretty good as far as fit goes, having a solid post and hole to give the glue more flat surfaces to adhere to. This can be tricky though – if the post and hole end up different sizes, you have a problem again.

But as hobbyists, you and I both know that sometimes the pieces just don’t fit together well. What solutions do we have? A lot!

Damn the research, glue it anyway.

This works well, if you never need to transport or drop your models on the floor. You’ve probably spent most of your life putting glue on a model, putting the other piece on the glue and holding it for a bit. Sometimes it doesn’t work out great, but if you have patience it will always hold eventually. Sometimes you have a point you have to glue, sometimes you have irregular pieces. More glue will work! …for a time. I’ve done this a lot. Let’s call this the “prayer” method, with all of the connotations that go along with that name.

Grab your hobby knife and file.

Certainly an option, you can cut your model up until the two sides fit. If you have steady hands and a good eye for 3 dimensions, you can create your own flat surfaces to ensure that the pieces all fit together. I’m really bad at this, and tend to make the problem much worse.

Glue:Filler:Glue.

An idea which I’ve heard about recently is to use something in the middle to allow the glue to adhere to. From the last post, we know that the problem with globs of CA (aside from the fact that the word “globs” just sounds awful), is that it can take longer to cure, sometimes will never fully cure, and when it does cure, it creates a weak crystalline structure in the joint.

So instead, you roll up a batch of putty, put a small amount of glue on one side of the joint, a small amount on the other side of the joint, drop the putty ball in the middle and squish. The glue will adhere to both sides of the putty, the putty is malleable and will change shape to fill the available space and after it dries you have a solid object in between two thin layers of glue. That sounds alright!

Liquid vs Solid Putty

I would use the solid stuff. The liquid feels more like it’s good for filling tiny gaps after you’ve glued. It doesn’t “squish” in the same way, and the squish is important to making sure that you get a filled joint. As well, the liquid is meant to be applied with a paint brush, and you’ve got CA in that mixture and if you do this you’ll never be able to use that paintbrush again!

Excess

After the squish, you’ll almost certainly have some putty sticking out of the edges of your joint. You can try to remove it immediately with a putty tool, but that CA will get in your way and all over your tool. It’s still an option, as you can clean that tool easier than the paintbrush! Also, while the putty is still uncured, you risk taking some of your structural putty and moving it out of a useful position or putting holes into it.

You can also leave the putty/CA combo to dry and clean it up afterwards. Most two-part putties can be cut and sanded after curing. Be gentle though – when cured the putty is a flexible substance (so when transported it may flex and bend, rather than snap. This is a good thing!) and if you put to much pressure on it, it can still come apart. If this happens, clean it all up – the putty, the CA, all of it – and try again.

After you’ve cleaned the edges of the joint up, depending on your skill, you may not have left the best looking area. My last attempt was a little ragged. I watered down some of the liquid green stuff and painted it onto the now dry area, which smoothed the whole thing out.

I’m using this method on my plaguebearer conversions, so I’ll see how it turns out. I suggest it in the first place because my hobby hero, Mr. Wappel, recommends it, saying:

Now for the wings.  Once again, glue /gs/ glue.  First wing in position.

The idea behind this technique is twofold.  First, it fills in some of the gaps that can happen in these kids of joints where pins are not always the best idea.  It also means that I don’t have to sit there holding it in place as the glue sets!  You can see the bead of glue in this image, with the green stuff on the inner wing surface.

I’m fixing and converting some second hand models tonight and wanted to toss this out there, since it’s on my mind.

When you use super glue, use as little as you possibly can.

I tried to do some research to grab a second source on why this is, but it seems that modellers aren’t the target market for most cyanoacrylate (CA) glues. There are lots of other applications, but they don’t seem to care how much you use!

My first guess at why you use as little as possible was that because it has a very poor shearing strength and because it’s very brittle, the more you use, the more weak link you’re adding to your model. (in a perfect world, your model would just be entirely plastic/metal/resin, with no glue, which would be much stronger of a join!) The problem with this idea is that it really shouldn’t matter how much you add, since adding any is weakening.

I came across an article that finally gave me a decent answer.

The cyanoacrylate glue hardens very quickly when trapped between two surfaces. The reaction is caused by the condensed water vapour on the surfaces (namely the hydroxyl ions in water). The water comes from the surrounding air, so obviously the air humidity is a factor that may affect bonding capabilities, or cause them to differ from application to application.

The curing reaction starts at the surface of the bonded material and develops towards the centre of the bond. Because of this, thick seams or large blobs of glue may harden less satisfactorily than surface-to-surface bonds with good fit. In a thick blob of glue, a polymerisation reaction may stop before it reaches the centre of the blob. A rule of thumb is that seams thicker than 0,25 mm should be avoided. Thick seams will also take longer time to cure.

The other thing I’m thinking about (not a chemist), is that CA joins two things because of a chemical reaction between molecules, in particular the moisture on another object. But the stuff in the middle crystallizes, and rather than creating a strong bond between molecules creates a lattice of CA molecules that just isn’t as strong as if you have moisture:CA:moisture molecules all touching.

Regardless of the science behind it, we all know that thick applications are just more brittle and break easier. Don’t do it, you’ll thank yourself when your model stays in one piece in your case when you’re traveling by plane! (baggage handlers…*shudder*)