My wheels nuts are coming loose.I had the original wheels shot blasted then powder coated grey/silver and they were ok until I decided to change colour. As the wheel nuts aren't like modern ones with a conical design I fitted spring washers which dug into the powder coating.so I sanded down where the nuts tighten up and sprayed them white. When I was out the other day 2 days later something started knocking,turns out the wheel nuts had come loose so I re-tightened them and they're starting to loosen again. I have cleaned the hubs and rear of the wheels.
Are they fitted with washers or should I put a bit of "lock thread" on them? I am cuatious as I had 2 snap whilst taking them off as I'd put lock thread on(maybe I overdid it)

You definitely should NOT put loc-tite on the wheel bolts! The torque for tightening the wheel bolts is:--32.5--39.8ft.lb/4500--5500kg.mm. I realise that you are using 'flat' bolts and not tapered, but with tapered nuts/bolts there should not be any paint between the nut/bolt and the wheel--I have seen the result of a Ferrari wheel that had been colour-matched (I think that it was 'powder-coated) and the area on the wheel that the bolt tightened up to had been left with the 'powder-coating' finish still on it. The P-C finish prevented the wheel-bolt from connecting to the wheel--the bolts came loose--1 (as it turned out!) very expensive wheel bouncing down the road!! try leaving the area where the washer goes 'bare metal'

the hobbler said:

You definitely should NOT put loc-tite on the wheel bolts! The torque for tightening the wheel bolts is:--32.5--39.8ft.lb/4500--5500kg.mm. I realise that you are using 'flat' bolts and not tapered, but with tapered nuts/bolts there should not be any paint between the nut/bolt and the wheel--I have seen the result of a Ferrari wheel that had been colour-matched (I think that it was 'powder-coated) and the area on the wheel that the bolt tightened up to had been left with the 'powder-coating' finish still on it. The P-C finish prevented the wheel-bolt from connecting to the wheel--the bolts came loose--1 (as it turned out!) very expensive wheel bouncing down the road!! try leaving the area where the washer goes 'bare metal'

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I presume washers are required to prevent the nuts chaffing whilst doing them up?

I think that Tom hit the nail on the head when he mentioned the required torque for the wheel bolts. I suspect that the wheel bolts are old and have probably been over tightened. The bolts on my car were stretched almost to destruction when I got it, necked in like a metallurgy test sample. The deformed thread will not do the female thread on the drums any good either so I would suggest new wheel bolts.

The spring washer that is put under the head of a wheel retaining bolt (a) 'loads' the bolt, which helps to stop it unwinding and (b) if the bolt does start to unwind, the 'barb' of the spring washer as the pressure on it is released will catch the head of the bolt and also help prevent it unwinding any more. It is wise to put a new spring washer on the bolt (if possible) whenever you remove/refit the bolt.

the hobbler said:

The spring washer that is put under the head of a wheel retaining bolt (a) 'loads' the bolt, which helps to stop it unwinding and (b) if the bolt does start to unwind, the 'barb' of the spring washer as the pressure on it is released will catch the head of the bolt and also help prevent it unwinding any more. It is wise to put a new spring washer on the bolt (if possible) whenever you remove/refit the bolt.

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Hobbler so you're saying that there should be spring washers under the head?
My torque wrench is broken, so I hand tighten them.but they get to the point where I feel they are going to break.

Nice heavy-duty spring washers. If you use the Fiat toolkit wrench or a standard length spanner or socket-wrench, you should be tightening just as much as you can do comfortably with one hand. If they feel like they're failing either you're overtightening or they have been so previously and threads are failing or the setscrews are stretched.