Repairing 'crazed' paint

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gregh
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Repairing 'crazed' paint

Post by gregh » Tue Feb 12, 2019 1:37 am

I'm always having problems using spray cans of paint on styrene.
Sometimes - just sometimes, the paint crazes like this
crazed paint.JPG
In this case I sprayed directly onto the bare styrene. Usually I spray grey primer first and it 'rarely' crazes.
I've searched for the reason, but drawn a blank. I've tried ensuring the can is warm enough (not a problem in Sydney in summer).
I've wondered if it is high humidity (it IS in Sydney). I've washed the styrene parts in soap and water and rinsed well but still SOMETIMES it crazes.

And usually the solution is to sand it all back and hope it works next time. Bit of a pain if I've spent hours adding rivet detail!
Anyhow, I have managed to find a part solution - I took a cotton tip (?) and dipped it in turps and washed it over the crazed paint. It dissolves and makes the paint 'flow' into the cracks.
repair crazing (1).JPG


the picture looks worse that it is - just use lots of turps so it 'flows'.
repair crazing (2).JPG

Not a perfect job, especially if you want a perfect finish.
But in this case of the bogie, it allowed a quick fix. Here's the result.
crazed 3.jpg
Greg from downunder.
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Re: Repairing 'crazed' paint

Post by Tropic Blunder » Tue Feb 12, 2019 7:50 am

Excellent tip!

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Re: Repairing 'crazed' paint

Post by philipy » Tue Feb 12, 2019 8:57 am

Greg,
I'm not trying to be clever, but when you say turps, I assume you mean 'white spirit' not actual turpentine proper?

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Re: Repairing 'crazed' paint

Post by ge_rik » Tue Feb 12, 2019 9:19 am

Handy tip, Greg. Thanks.
This has happened to me a couple of times. On both occasions, it was when the coat I was applying reacted badly with paint which was underlying it. Very frustrating, particularly when it's on a loco body with a lot of rivet detailing, so almost impossible to rub down without having to remove and replace all the rivet details. If it happens again, I'll give this technique a try.

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Re: Repairing 'crazed' paint

Post by gregh » Tue Feb 12, 2019 10:16 am

philipy wrote:
Tue Feb 12, 2019 8:57 am
Greg,
I'm not trying to be clever, but when you say turps, I assume you mean 'white spirit' not actual turpentine proper?
Here in Oz, it's full name is Mineral Turpentine, but called turps by everyone. It can be used to thin enamel paint and for cleaning brushes.
Don't know what white spirit is.
Greg from downunder.
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Re: Repairing 'crazed' paint

Post by GTB » Tue Feb 12, 2019 10:19 am

philipy wrote:
Tue Feb 12, 2019 8:57 am
I'm not trying to be clever, but when you say turps, I assume you mean 'white spirit' not actual turpentine proper?
Depends on what you lot call 'White Spirit'.........

'Turps' is 'Mineral Turpentine', a petrochemical solvent, in common use here in Oz for thinning enamel paint and cleaning the paint brush when you finish. As you say, not the same as the 'Turpentine' used by artists, which is distilled from plants. Alhough 'turps' is sometimes labelled as 'Turpentine Substitute'.

Here in Oz 'White Spirit' is a different petrochemical solvent with a higher flashpoint and lower aromatic content, sold for household use as a dry cleaner.

I keep a bottle of both in the workshop, Turps for cleaning paint brushes and White Spirit for use as a degreaser.

The high aromatic content of turps means it is a polystyrene solvent. If your 'White Spirits' can stick polystyrene together, it should do the same job as the turps described by Greg.

Graeme

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Re: Repairing 'crazed' paint

Post by philipy » Tue Feb 12, 2019 10:48 am

Thanks guys. i think we are all on the same page. It used to be called "Turps substitute" here. However these days there are products called "Low odour white spirit" which smell different and don't have as vicious effect on plastics,so might not be as effective in this application.

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Re: Repairing 'crazed' paint

Post by GTB » Tue Feb 12, 2019 11:30 am

philipy wrote:
Tue Feb 12, 2019 10:48 am
However these days there are products called "Low odour white spirit" which smell different and don't have as vicious effect on plastics,so might not be as effective in this application.
We've got something called 'Low Odour Turps' which is much the same as the 'White Spirit' sold by the same company. OK as a degreaser, but not my first choice for cleaning paint brushes. Marketing Depts in action........ :roll:

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Re: Repairing 'crazed' paint

Post by GTB » Tue Feb 12, 2019 12:14 pm

gregh wrote:
Tue Feb 12, 2019 1:37 am
In this case I sprayed directly onto the bare styrene. Usually I spray grey primer first and it 'rarely' crazes.
I've searched for the reason, but drawn a blank.
As Rik reported it is usually due to the solvents in the top coat attacking the layer under it. There are paint systems available deliberately designed to give the same effect for instant ageing of furniture.

The grey primer you use is probably more resistant to the paint solvents in the top coat than polystyrene is, especially once the primer is fully cured. The primer then acts as a resist layer and stops the polystyrene underneath from being attacked by the paint solvent.

How much cracking you get would depend on how wet the top coat goes on, what solvents the paint manufacturer used, as well as how well the primer had cured before the top coat was applied.

Regards,
Graeme

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