Dealing with cracks in Zamac

[Tinman]

Epoxy glues, they are not all the same.  Take JB Weld for example, they have an entire product line with such examples as the "Steel" dark grey two part epoxy, a clear two part epoxy, a putty epoxy with pre-added (unmixed) two part components and super glue type products (Polyisocyanates).  Of note, is the various one part products that are made for various plastic mending uses and a putty two part that can be used on PVC and ABS.

Most modelers and restorers purchase the two part dark grey "steel" epoxy. It works well for most but far from all uses. However, experience has taught me that it is not always the best for all purpose use with diecast model repairs. You can watch and read all the online reviews (most of which are sponsored by JB Weld) and come away with the feeling that their products can fix anything.  This is partly true, but you have to use the right product for each repair. You can't purchase the two part "steel" JB Weld and expect it to work on all projects, large and small or different materials.

In photo's below you will see a Kingsize Matchbox tractor with half of the tow hitch broken off. There are some good ways to fix this problem: Muggy Weld, UV light cure bonding/welding glue and epoxy.  My first impression was to tackle this with JB Weld "steel" epoxy.  It failed on two repeated attempts. The epoxy just could not obtain a strong enough bond with the tiny area of initial contact. Since I don't have the UV system and I'm all out of Muggy weld sticks, I had to chose another epoxy.  But first I'll mention why I went with JB Weld "steel" on the initial tries. 

Viscosity, it's got to be built up and curved into the shape of the hook (a challenge in itself). The JB Weld "steel" has a pretty good (thick) viscosity. I wanted to build it up free hand without assistance of a mold, so this was my first choice.
It failed quickly on two attempts. Every time I tried to tool the epoxy, it simply broke away at the contact point of the broken hitch.  I concluded that the heavy viscosity made less of a bond at the tiny critical interface.  I switched to a lighter viscosity clear epoxy. This made the building up go slower and required multiple applications.  It became a side project, every time I mixed up a little clear for a project I used the excess as another application to the tractor hitch. The result was a success. The clear epoxy was well bonded and was well cured due to the longer cure times (since it became a side project).  I was able to tool the epoxy without it breaking off at the tiny point where it joined the hitch. 

I wish I had the forethought to take additional photos. Like many of us, once I'm "in the zone" I just work straight through. Not being a chemist, I have my own thoughts on why they bonded differently. I do know a fair bit about viscosity and how the "wetting in" issue affects the chemical bond of various coatings and compounds.  My short answer is that I suspect the thicker viscosity of the JB Weld "steel" didn't make a strong enough bond on the molecular level as it could not cold flow into all the tiny nooks and crevices. For all my small jobs I (now) never use the dark grey JB Weld and go with the clear two part epoxy.

Also shown is the two part clear epoxy I have switched to. I dislike the JB Weld clear in the syringe as (once opened) it has a short shelf life and you have to extrude a fair amount of it for each use.  With the smaller individual tubes, I can mix smaller quantities and even add more hardener for a faster set time. The screw caps give a longer shelf life after opening. I usually can use up the entire contents of the tubes over time without issues. Before I close, another way to reform the tow hook is to make a clay mold using a whole tractor's tow hook. Place the broken hook tractor into the mold and fill the open part of the mold with two part epoxy (standard resin molding practices must be followed for proper release). The only other tractor I had has the later issue "Square hook" so I could not do this and had to free hand the build up.

[Tinman]

Apologies in advance for taking up too much space and time on this thread. With regards to filling small cracks there is another way besides the use of the one part glue I mentioned in a previous post. There is an old trick to give paints more viscosity and better coverage (it works for latex, acrylic or solvent paints). There used to be a commonly available additive for paint called "whiting." You added it to paints to make them thicker and to cover better. These days, it's nearly impossible to find whiting and it's not really needed due to modern paint chemistry. Quality paints have super fine ground additives (titanium dioxide being the best one) to increase viscosity. These give paints better viscosity and improved coverage without ruining the "leveling" of applied paints (leveling is the ability for the paint to lay out flat and not show brush strokes or roller "orange peel).

One can make their own whiting by adding baking soda or baking powder in small amounts to paint or primer. You can then use the modified paint to paint over and fill hairline and small cracks. If you add it to primer it sands very easily to remove any excess. If you use the "whiting" sparingly, the paint lays out flat and it hides those imperfections quite well. Since, in many cases, you are making a cosmetic repair and not a structural repair there is no need for anything stronger. So these simpler methods will be faster and easier and eliminate the need for shaping, scraping and lots of sanding.