Techniques for assembling metal models

[Russ]

Area for techniques on assembling 1/144 diecast metal models.

Articles needed.

[pbhawkin]

I have only built a couple of Jacques' White metal planes (Creation Chaubet site in links section) but CA (superglue) seems to be the only way of glueing them. 2 part epoxy (araldite) may also work BUT with it's long setting time I find is a pain to work with generally.

[smeg1959]

I agree with Peter's comments on adhesives for white metal/pewter kits. Whilst potentially giving the strongest bond between model parts, 2-pack epoxies are not very feasible because of their limited working time after mixing. There are single-pack epoxies, but these tend to be specialised and not really suitable for small scale kit construction.

That leaves CA as the "go to". I would add that the viscous form of cyanoacrylate (the one with the consistency of polystyrene adhesive) is the one to use. Whilst I find it excellent with plastic kits, liquid CA doesn't bind strongly enough quickly enough on metallic substrates.

[smeg1959]

Whilst obviously not confined to metal models, I'd like others' thoughts on assembling brass "bits and pieces". I've got it in my head to build the fairly simple beaching gear used to wheel the flying boat version of the Catalina around (there's a decent sketch in the Squadron Signal "In Action" issue covering the type) and have appropriate diameter brass rod and aluminium tubing but am questioning whether I take the Superglue/CA approach or look at soldering where one piece of rod needs to be attached to another.

Incidentally, if anyone is wondering why I'm not going down the plastic rod route, I should add that the beaching gear will be attached to my Altaya/IXO metal diecast PBY-5 (the RAF 209 Squadron version) ... definitely too heavy to trust to thin poly pipes.

[Russ]

Wanted to see if anyone else had more experience on these. On small metal parts like tubing and for brass sheet, I have used both glues and solder. For metal to metal I have had best results using solder when the metals are copper and brass but not with aluminum or steel. From my electronics tools I have a set of various soldering irons and will usually go with the finest tip and use finer size diameter solder with a core of flux material rather than brushing it on. These principles can also be of use to anyone building illumination electronics into diorama items.
First is to have the parts clean and I prefer them roughed up just slightly, like with 600 grit (or a good clean pencil or ink eraser) where the joins will be soldered. I'll use a 10 watt needle pencil type iron for finest wire and PE type work and the bigger 35 and 70 watt for the big stuff. I don't use a torch, was never a plumber! Remember that you are applying heat to not just the join area but the entire part as metal is a good heat conductor. So even if joining a thin wire to a big metal sheet, the 35 watt iron is the way to go because the big heat sink sheet will suck away heat from the join area until all of it is hot or becoming hotter. Use a clamp stand to hold your work and practice safety with the hot iron, remember that it can start a fire if touched to things in your workshop and really deep second degree burns to skin. Use some solder to "tin" the tip of your iron because a dry or dirty tip is actually a bit of an insulator and will transfer less heat to the work than a liquid metal covered tip. Keep a wet sponge in a bowl to wipe the tip on while working to get off all of the dirt, slag and debris that builds up on the tip. The tip should be clean and covered in a light coat of solder to work at its best. Touch the tip with a bit of pressure to the join area and in a second or so, start applying solder to the tip and join area. Watch how the solder is behaving, if it is flowing onto the tip and not the work, then the items being joined are not hot enough yet. Wait a few seconds and try applying again. Ideally you should see the flux material flow with a bit of boiling and spatter, onto the work and then followed by the silvery solder, which will bond to the surface and start to flow over it. You don't want too much solder, just enough to create the join at the immediate contact area and not travel very far from that site or start to mound up. If you do apply too much solder, there is a product called solder wick, which is braided thin copper wire ribbon, which will by capillary actions, draw off excessive solder from a heated area, often taking all of it away if you apply it for too long. If you desolder a join area with wick material- just resolder it again like starting from scratch. The flux acts as both a cleaner and a material that helps in the transfer of heat, don't worry about the residue of this material left on the join area, this is normal and can be cleaned away.

A good idea is one I was taught to do 40 years ago as a budding electronics Engineering Tech. Get some old circuit boards from junked radios and from electronic scrapped appliances and practice desoldering components from them. You can learn to use needle nosed pliers to grip parts you are trying to pull when the solder becomes melted or to hold down parts you are bonding into place. Try soldering some old copper wires to each other and then to a copper circuit board to get the feel for how fast copper foil, wires and flat parts will heat up to the point of accepting new solder or softening up old solder enough to pull out a part. we did this at school to also build up a stock of spare parts. This was in the days before "disposable" technology. Doing this will give your fingertips the feel for when a solder joint releases and for when it solidifies after removing the iron. Do this on junk items before starting on your project parts. My instructors had 3 categories of soldering they would critique us on.
Technician solder- Joint cleanly established, just enough solder, all in the right place and fully melted then cooled, part placed so that it was a pretty good fit and hold even before any soldering, rather than just looking tacked on.
Engineer solder- Joint made, probably too much solder or some unsoldered area showing, not enough used, and lots of dirt around the join which was skin oil and other loose stuff melted into a black froth by the operation (you can clean it away after the work is done with alcohol and a small wire brush), and the part really mechanically held by just solder itself.
Manager solder- Looks like solder was dropped from 6 or more inches onto the work, circuit board burnt in places, fingers burnt in places, colorful debris on the join area, colorful language used throughout.

Probably some good videos on youtube on how to do good electronic soldering on small parts too.

I would size up a project and if there are maybe just 10 or less joins to be done, go with CA versus getting all of the kit together for soldering.

[smeg1959]

Thanks for that, Russ. I do now recall that aluminium (or, for our US readers, aluminum) doesn't respond well to conventional soldering techniques. The reason is that the aluminium itself is very reactive, so much so that it combines with oxygen in the air to form a thin coating of aluminium oxide, otherwise known as alumina. The alumina is a poor conductor of heat and interferes with the soldering process.

If somebody desperately wants to solder aluminium, there's some useful info on the website of American Welding Society (see http://www.aws.org/wj/2004/02/046/). However, I think I'll go with CA for the aluminium tubing and reserve the solder for the brass-to-brass joints.

[smeg1959]

After a few trials with painting some WGS tanks (described elsewhere in the forum), I thought it prudent to include a couple of comments here.

The first regards priming the assembled kit or individual parts prior to painting. From personal experience, acrylic primers like Mr Surfacer work wonderfully well on various plastics, including resins. However, whilst they spray smoothly onto white metal, they are prone to chipping off once dry. In other words, they don't form a strong enough bond with the surface of the metal.

Enter etch primers. These coatings generally contain phosphoric acid which etches the metallic surface, forming microscopic pores that allow the primer to "key" to the metal. The upshot is that the primer forms a bond with the metal that is resilent enough to withstand scratching or chipping. Now I should point out that these are not designed for priming white metal models but, rather, large items like car bodies, metal frames or buildings, outdoor furniture frames, etc. However, they generally spray thin enough to prime models without obscuring fine details.

Depending on the brand, it is possible to find etch primers that range from silver through grey, red and even yellow. The red primer usually contains iron(III) oxide whilst the yellow has zinc chromate as an anti-rust agent. My experience has been with one of the grey primers purchased at a local hardware store (for the record, a product called Dy-Mark Etch Primer, manufactured in Australia). It touch-dries in under 15 minutes and results in a smooth thin coating that is similar in colour to a medium grey.

My second point concerns removing paint and primer from white metal items ... simply soak in acetone. Agitating the items in this solvent will be enough to remove most, if not all, of the coatings, leaving you with a pristine unpainted model ready for your second attempt at finishing! Indeed, I now pre-clean any white metal casting with acetone before priming to remove any greases or other contaminants from the surface.

A warning though. You should never use acetone on plastic items. Whereas solvents like isopropanol and those compounds in model paint thinners are perfectly OK, acetone will start to dissolve the plastic then moment you apply it. At best, you will roughen the surface which will then need to be sanded. At worst, prolonged exposure will distort parts and remove plasticiser, making the plastic extremely brittle.

[smeg1959]

One thought re: etch primers. Some of you may have decanted paint from an aerosol can and used it in your airbrush so that you get much greater control and substantially less overspray, as suggested in a recent Fine Scale Modeller mag. Not that long ago, I did this with Tamiya's AS-12 spray when completing a Minicraft Mustang and it worked pretty well. Now, given that you could conclude that using a hefty spray can of etch primer designed for covering large surfaces like car bodies is overkill on a 1/144 tank or aircraft, your thought might be, "Hey, why not decant this stuff and use my airbrush instead?" Just one problem. Etch primers etch metal. What are the "guts" of airbrushes made out of? I rest my case, Your Honour.