does anyone know how I can open or loosen a metal screw cap of a bottle of polyester resin which is stuck with the resin? Last time I used this resin was 3 months ago and I wiped the mouth of the bottle as usual. I tried to put the bottle upside down and soak the cap part in hot water, and also put the bottle in the freezer for a couple of minutes but the cap doesn't move at all. Thank you.
If the resin has completely set, you may need to break the cap or bottle to get out the remainder. However, it may just be extremely viscous, so you could try slow, steady force with a rubber band wound around the cap to get a better grip (squeezing with pliers will just break the container).
Gently heating the top in hot water may also reduce the viscosity, allowing the rubber-band trick to turn it.
There are also rubber strap-wrenches, but they might also crack the bottle, though less likely to do so than pliers.
For the future, put a bit. Of Saran Wrap over the bottle top, and then screw on the closure. It will now be real easy to remove the cap.