I found some old Tins of Airfix and Humbrol Paints

[BillMC]

Unfortunately the paints have went hard - is there anything I can add to them that will liquidfy them? Such as thinners maybe?

[Ratch]

I presume these are enamels.

[peebeep]

Probably not worth trying to revive them but you could try Mr Color Replenishing Agent.

 

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[zuludog]

It depends on how hard they are - if they're just a bit thick you could thin them with white spirit or enamel thinners, but it could be hard work as the paints had quite a lot of sediment/pigment even when they were new.

If they are very thick they will need not so much stirring as first breaking up with a steel rod or a small screwdriver, followed by a thorough and prolonged stirring & mixing

 

If they are very hard then I'm afraid there's nothing much you can do except to throw them out

If you want to re-use a few tins for your own mixtures you can scrape out the sediment and clean them out with paint stripper then white spirit

 

I don't know the technical explanation, but as the solvent evaporates the paint will become thicker, but eventually there will be a point of no return where you can't recover them, and you just have to admit defeat

 

If you do manage to recover a few I would try them on scrap before using them for any proper modelling

 

However, if the tins are well sealed and don't dry up the paint has a very long shelf life

A break of many years due to work & family means that I'm a returnee, and some of my paints are in good condition even though they've been stored since the 1980s, possibly longer

[BillMC]

I presume these are enamels.

Yes they are.

[Ratch]

I never had great success trying to restore enamels. I've tried with white spirit in the past. Maybe toluene or M.E.K. would do a better job.

[peebeep]

You need a very hot thinner to break down the pigment. Even if you can get the hardened pigment back into solution it's likely to be very grainy, with poor adhesion - I can tell you that from personal experience. The GS replenishing agent is a very hot thinner, plus it has a resin binder added so that the revived paint still sticks. It's designed for acrylics and I have no idea if it will work with enamels. My experience with enamel is that if it's hard in the tinlet, then it's only fit for the bin.