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Tips and tricks for UV curing glue, resin and coatings Answered

Only a few years ago your only option to repair certain plastics, glass or even a broken crystal was epoxy based resin or the good old superglue.
You might have already tried one of the 5-seconds-repair pens or tried your own UV curing nail polish art at home.
For the later you might be lucky as the resins used here are optimised for the purpose and lights you get with them.
Sadly even the best nail polish is no substitude for a glue as the material properties need to be different.

One of the most common complaints when it comes to using some UV glue, like Kafuter or similar is that it never comes with instructions.
Sould be straight forward but it is not free of problems.
For example almost all commercail UV curing glues that you can buy require quite stirct procedures and for the light the right wavelenth(s).
Resins and coatings can be even more painful here as they might also require you to stick to the correct temperature.

Let's start with one thing you might have encountered already...
The glue is definately cured and rock hard but the surface tacky and smeary.
Quite annoying if you want to fix a piece of jewellery and can't prevent it from collecting dirt and dust...
The next thing you might have encountered is that despite having transparent materials it seems to be impossible to cure the clue.
Both problems come down to wavelenght and exposure.
UV curing glue is prevented from curing in the presence of oxygen - a factor utilised for example in resin based 3D printers.
Uncovered glue is exposed to the oxygen in the air and won't cure easy.
The glue or resin below this layer however with fully cure with ease in the absence of oxygen.
For the second problem consider that not all materials that you can see through will let UVC light pass through ;)

Bonding strenght is another complaint I hear a lot...
Be aware that certain things just are no good for UV curing glues or resins.
Take the molds you get for that purpose: on the material the glue won't bond!
Teflon is another prime candidate here.
But in a lot of cases it comes down to surface preparation.
Don't be afriad to sand the surface!
Not only will the surface area increase but the scratch marks will be invisible once filled anyways.
Use sandpaper on your fingernails, then go over with clear nail polish -mirror finish ;)
With curing often a problem consider to fully cover the glue.
A bit of clear sticky tape, food wrapping foil....
If that is not an option then eliminate the oxygen.
You can use a container filled with inert (for the glue) gas like CO2 or just place a burning candle in it until it goes out....
Either way the amount of oxygen should then be low enough to cure the surface of your glue.

Not always is any of the above an option.
Then you can still try more power and a lover wavelength.
Mercury based lamps for example provide a very broad and powerful light that in most cases will cure within seconds.
For a proper surface cure you need a wavelength of 265nm or lower.
LED's offering this exist but at prices well out of range for the hobby user.
A mercury lamp under high pressure is nothing for short term use and the limited lifespan does not always justify the costs of buying them.
Like with most things in life certain inventions can have a dual purpose.
Quality germicidal lamp systems for examples often state to go as low or even lower than 265nm.
And they come at a fraction of the cost you have with a broadband mercury lamp.
Even cheaper is the fre weather forecast.
If the sun is siad to be strong enough so you need protection than even the worst glue will fully cure in seconds outside in the sun - tackfree!

Don't be fooled and protect yourself!!
These tiny LED lamps for your glue stick, the curing thingies for your nailpolish and everything else using UV light comes with warnings.
For very good reasons!
It might be hidden in the fineprint but you can not really see UV light.
The blueish-purple glow you see is on the high end of what comes out and by that in the visible range of your eye.
Just because a LED only gives a faint glow you see does not mean the UV light wouldn blind you if you could see it!
Even worse for fluoroscent lamps or open cruning systems like those for your nailpolish.
Reflected UV light is still UV light and you can still NOT see it!
Stories of people getting sunburnt from germicidal lamps in a butcher shop or other people going blind from checking money as their living have a true base...
In most cases lamps used well past their lifespan or simply the wrong type of lamp but still: the damage came from UVC light...
If you just love creating your own artwork or jewellery with UV curing resins and glues than protect yourself.
Proper sunglasses with a stated UV protection for example or just black nitrile gloves for your hands...