I have an old tin of fibreglass resin that has formed a very thick skin(because I suppose the tin was not air tight) so that very little was left that was liquid and so useable.

I have tried an expedient of first air tightening the tin and then adding an amount of acetone that I happened to have.

Can I hope that ,in time the resin (which has not been in contact with the hardener but hardened on its own) will combine the the acetone and become useable again?

I don't mind waiting up to a year as I can purchase a new tin of resin to use for now...

Edited August 23Aug 23 by geordief

Some resins are activated by amines, some by UV, some by oxidizers.
IOW, air can be a self activator.

The activator is an 'accelerant' that speeds up a slowly occurring chemical process.

58 minutes ago, MigL said:

Some resins are activated by amines, some by UV, some by oxidizers.
IOW, air can be a self activator.

The activator is an 'accelerant' that speeds up a slowly occurring chemical process.

One step forward ,no step backward :-(

1 hour ago, MigL said:

The activator is an 'accelerant' that speeds up a slowly occurring chemical process.

Be careful to distinguish between a catalyst, which is not present in the products of the reaction, and an accelerant such as lighter fuel which accelerates fires (oxidation) and is used up in the reaction.