I began the preparation by washing the two chosen un1tec barrels in a soap bath, then using some other cleaning compounds to get the remains of glue out of the surface of the silver one.
Put them in an electrical oven with temperature set to 250 degrees Celsius.
Checked on them again after 40 minutes - The black barrel changed color into a nice milk chocolate brown, while the silver one didn't seem to be affected at all... But wait a minute... there are smears on both barrels, with some glue remains leaving a slight blemish on the silver one's surface (not to worry, it will be covered up by the thread back anyway) - quickly removed them, cooled them, washed them again and then put them back into the oven for another hour to see if there's any change.
Below you will find picture showing the barrels before I put them in and when I checked on them after about 40 minutes.