This is the exact problem I have. I have tried putting the Fujik into a syringe tube and it still hardened inside!
Any suggestion for keeping the Fujik?
Fujik is much cheaper if you buy it in bulk at the hardware store. Look for white bathroom caulk, it's the same stuff and works just as well (i.e. not very).
Fujik is ok for mounting generic 1W LEDs (16mm or 20mm mcpcb)
Its thermal conductivity is not perfect. little higher than regular RTV silicone.
I used thermal paste for mcpcb and then glued it with regular epoxy (edge) and had decent results. but it not easy. you have to press mcpcb to seat it well and remove excess thermal paste.
another method is mix thermal paste with epoxy. I don’t know ratio. Once I glued small heat sinks to RAM chips of VGA card. 50%/50% mixed.
My method of Fujik or other RTV silicone keep without hard:
After use, press tube again to come little silicone to out then cover hole with piece of tape and gently wrap tape and then put cap.
Reason for get silicone to out is remove any air between tape and hole.
no air=no moisture=no dry. I keep silicone tube in fridge. (here room temp=30C)
after some storage time, it will hard near hole. but not much. you can easily remove hardened silicone clot.
Calcium carbonate added to epoxy gives a somewhat better thermal conductivity. I have used a quick and dirty method using dried eggshells ground super-fine in a small coffee grinder mixed with epoxy to fill a hollow pill and it worked decently. Not as good as commercial products, but it was OK if the powder was fine enough. I know it sounds strange…
Do NOT substitute regular silicone caulk for Fujik.
Regular bathroom caulk contains acetic acid and it will etch pcb's. Most hardware store RTV silicones contain acetic acid. They are NOT safe for electronics.
The clear, maybe, though it's becoming less common. I don't think I have ever seen a non- low-volatile white silicone caulk.
One of the few easy-to-find silicones that does not contain acetic acid is GE Silicone II.
Every other hardware store caulk that I've ever bought, clear, white, brown, or black, all contained acetic acid. Even the Permatex High Temp RTV I got recently contains acetic acid.
But before anyone goes out and buys GE II silicone, GE itself recommends "that consumers not use silicone caulk in any electrical application".
How many times are you going to edit that? Sheesh.
A few more times, probably.
They don't contain acetic acid per se, it forms as a by-product of the reaction with water in the air that causes curing.
Thanks for correcting.