Wait, wait, just hear me out first.
Outstation boots are built with cemented soles. That is, the leather midsole is stitched to the rubber slipsole, which in turn is cemented to the outsole.
Learning how to use contact cement correctly, now this was the most difficult part of the bootmaking process to master. It was harder than patternmaking, lasting, everything.
You’d think that it should be pretty straightforward right? Take a brush, dip, apply contact cement, stick material.
Well yes, but mostly no.
And over the many years, I’ve come to realise that the processes of applying contact cement and makeup are disturbingly similar.
Don’t ask me how I came to this conclusion.
It starts with the right makeup.
So anyway. All of my mistakes using contact cement I learnt the hard way. Through some kind of partial sole delamination.
This is the worst nightmare any maker can have.
In retrospect, it was a blessing in disguise, because it forced me to understand the proper usage of contact cement.
It also kickstarted me into doing strength testing before rolling out a new cement into production, particularly on new materials.
The right makeup for the right skin.
So it first starts with finding the correct contact cement, formulated specially for materials commonly found in footwear.
Here in Malaysia, I can’t get hold of the typical cements used elsewhere, namely Renia and Barge.
So when I first started shoemaking many many years ago, I’d use the most common type of rubber cement, Dunlop, that was available in the market.
That was the first mistake. Contact cement isn’t the same as rubber cement. They’re used for different types of materials.
I guess this is the same mistake as using the wrong makeup for your skin type.
Once I realised my stupidity and found proper contact cement, my second mistake was the application process.
I, like an idiot, never actually researched how to use contact cement properly. I just assumed apply it to one side, then stick it to the other. Finished.
I can’t stress how wrong this was. Horrifyingly so.
I guess it’s the same with makeup. You can’t just take whatever out of the bottle and go smearing it willy-nilly. There’s a certain process to it.
And for contact cement, that process is:
- Apply a thin layer to both materials. Let it dry to a certain state. More on what this means later.
- Apply a second layer to both materials, let it dry again.
- Heat both materials to a specific temperature with a heat gun.
- Press both materials together. Add pressure. Manually clamp or run a welt press around to make sure the edges are tight.
- Leave everything alone for 24 hours. Don’t touch it.
Let’s now go deeper.
Caveat, I’m not a chemical technologist. I only explain what I’ve learnt from my understanding as a contact cement user (and now enthusiast).
Know how to apply your makeup correctly.
Firstly, contact cement works by using chemical bonds to achieve its outcome.
This means that contact cement only bonds to other materials coated with contact cement. No cement means no bond. Very simple.
When the two materials touch, the contact cement on each side chemically bond to each other immediately. There’s no second chance for repositioning.
This is different with other forms of glue, where it’s applied and then the materials are pressed together. The drying time gives wiggle room for any necessary adjustment.
There’s no wiggle room with contact cement. It’s because the cement already has to be dried to a certain level before it can bond successfully.
This is called the open time, where various solvents within the cement evaporate, leaving behind the parts of the cement that create the chemical bond.
If it isn’t left to dry and both materials are pressed together, solvent is trapped in the cement, leaving no opportunity for evaporation. Because of this, the materials won’t bond – even if both sides are coated with contact cement.
This open time however, also has an expiry. Too short, and the solvent won’t be fully evaporated; too long, and the remaining chemical layer will dry out.
So this means, whenever contact cement is being used, immediately I’m on the clock. Careful timing and process needs to be followed.
Before makeup, comes foundation.
I mentioned that I apply two layers of cement to each material. The first layer of contact cement acts similar to how a foundation does in the makeup process. It gives some kind of base for the makeup to stick to.
To be honest, I have no idea what foundation does actually, but I feel my theory makes sense.
Anyway. Especially on leather, contact cement absorbs into the fibre structure quickly. This is another reason that a second layer has to be applied on top.
Regardless of material, a second layer also acts as a safety net, making sure that I’ve covered every bit of the material in contact cement, because remember, if it isn’t coated, it won’t stick.
Prep the skin and keep it ready.
Normally in a two layer process, the first layer of contact cement isn’t so strict on the open time. Remember, that layer isn’t actually used for bonding purposes. It’s there as the foundation.
When I’m ready, I’ll apply the second layer of contact cement. This is where the clock really starts ticking.
After the second layer is brushed on, I’ll put the materials into the shoe oven for around 10-15 minutes at 60 degrees Celsius. This temperature accelerates the solvent evaporation and activates the contact cement ready for bonding.
It also puts the cement into a holding pattern, which lengthens the open time without drying out the cement.
I have no makeup equivalent process of this. Last time I checked, putting your head into the oven wasn’t a part of most makeup processes.
If a shoe oven isn’t used, the open time really depends on how the contact cement reacts with the surrounding environment, particularly humidity and direct airflow.
Only through trial, a lot of error and much testing, will I know how a particular brand of contact cement works best, in my own workshop.
Don’t touch your makeup after you’re done.
After the materials come out of the shoe oven and are bonded together successfully, the cement layer must cure to achieve maximum bond strength. Usually I leave my boots for 24 hours before I touch them again.
After applying mascara, you don’t want to rub your eyes immediately right. You’ll just disturb the setting process of your makeup.
For 24 hours, there is no outsole trimming, sanding or finishing taking place. During this time, the contact cement is hardening into a single cohesive layer.
Oh wait, I totally forgot.
Before the entire cement application process begins, there’s a priming process.
Sorry.
Before foundation comes cleanser. Or so I’ve heard.
If the first application layer of contact cement is the foundation, primer must be equivalent to a cleanser in the world of makeup.
From my understanding, cleanser should also be tailored to the type of skin it’s going to be applied on.
And it’s the same with primers. There are different primers chemically matched for different materials.
So in the case of rubber, I use a rubber appropriate primer to chemically etch the surface. This means that when the first layer of contact cement goes on top, it bonds better to the primer-soaked material.
Primers are not only specific to materials, they can also often be paired to the type of contact cement that’s being used.
So I apply a single layer of primer and let it dry fully, because similar to contact cement, various solvents also need to evaporate before it becomes effective.
I have no idea about open time for primers, I just wait until it’s dry to the touch before applying the first layer of contact cement.
Leather doesn’t need a primer, because it’s not a synthetic material. Instead, if I’m cementing a leather midsole, I’ll rough it up on the belt sander. That way there’s more surface area for the contact cement to grip to.
And this replaces what the primer does by chemically etching rubber, for example.
Wash your face with water before using the cleanser.
Taking another step backwards, even before using primer, it’s important that the materials are clean.
This means that rubber outsoles or slipsoles should be sanded lightly to remove mold release chemicals used in the manufacturing process.
Sanding however, leaves a layer of dust and other debris. This is what has to be cleaned off or the primer can’t penetrate into the material properly.
Acetone normally does the job well here. Wipe a few times, let it dry.
Don’t try using it on your face though.
Every makeup is chemically different for a reason.
Contact cements come in two major forms, grafted and ungrafted. From my understanding (and spectacular failures) grafted contact cement is stronger.
The main chemical used, chloroprene, has an additional chain of chemicals grafted to it during the manufacturing phase, improving how the cement reacts with materials and heat.
PU cement is a completely different type of contact cement. It’s used in a similar way to chloroprene based contact cements, but must be heat activated to bond correctly. Normally around 100 degrees Celsius.
Due to whatever chemicals are inside it, PU cement works very well with synthetic materials.
It’s incredibly strong, meaning it rarely (if ever) delaminates. The downside is that it’s almost impossible to remove. So, no recrafts.
If I try to rip an outsole off that’s been glued with PU cement, I’ll almost certainly destroy the midsole in the process.
Removing makeup also has a process to follow.
Like I was saying, I can’t find Renia or Barge in this part of the world.
So after much procurement effort, I managed to find a local supplier that has grafted contact cement; Lem-On Graft 655.
I also have a wide variety of matched primers for whatever midsole or outsole material I’m commonly using.
Because of this, my bench looks like an alchemists workshop. Glass jars scattered around with varying forms of liquids inside.
Anyway. Back to removing contact cement. This is more or less my process:
- Cut any stitches that need to be cut.
- Use a heat gun to reactivate the contact cement.
- Squirt a bit of acetone into the glue layer to help dissolve it.
- Slowly peel back the sole, midsole or whatever.
Makeup shouldn’t be permanent.
At times customers new to the heritage boot world will ping me this question.
‘Why is my sole peeling?’
And it’s a good one to explain here.
Firstly, heat and humidity. Asia in particular suffers from this. Heat is both the friend and enemy of contact cement.
Because heat can activate cement for both bonding and unbonding.
So trust me, leave boots in the car for a day and your sole may start to peel afterwards.
It’s no different to makeup. After a day of wearing it in the heat, it’ll also start to melt.
Secondly, contact cement – like makeup, isn’t designed to stay on forever.
I explain this to customers very simply. Enough glue has to be used to make sure that the sole stays on, but so much can’t be used that the sole can’t be removed later.
This is what immediately eliminates PU cement as an option for contact cement. Yes, using it likely means a sole will never peel, but that same strength means the sole can’t be taken off later.
The right amount, for the right time, for the right use. Just like makeup.
I want to use just enough contact cement with the right strength that the sole begins to peel naturally, just as it needs a recraft.
After explaining it like this, it’s a simple enough concept for customers to understand why a sole can peel, when it can occur and what it means.
Because remember, in a non boot-nerd world, a sole peeling means either there’s a defect, or it’s time to throw the pair out.
And that’s that. Around 1 year worth of trial and error, failures and research on contact cement and rubber compounds.
For those interested, the filled out process for applying contact cement is:
- Sand both materials lightly.
- Wipe with acetone to remove any loose debris or dust from sanding. Let it dry.
- Apply a thin layer of primer. Use the correct primer for the correct material. Let this also dry to the touch.
- Apply a thin layer of contact cement to both materials. Let it dry according to the open time. Going over this time is ok.
- Apply a second layer to both materials, let it dry according to the open time.
- Heat both materials to around 60 degrees Celcius with a heat gun. No heat gun is also ok, just be very precise with the open time.
- Quickly press both materials together. Manually clamp or run a welt press around to make sure the edges are tight. There’s around a 30 second window to perform all of this. Add pressure for another 60 seconds in a sole press.
- Leave everything alone for 24 hours.
And there you go. Just like applying makeup.
