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Old 31st Jul 2020, 3:28 pm   #21
stevehertz
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Default Re: What causes this in polyurathane varnish?

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Well, blast. I thinned the polyurethane with white spirit, got it nice and liquid like, no treacle. Applied with a brush as thinly as I could, and again I have a mess

I will have to try again and see if I can spread it thinner still
As I say in post 18, interaction with previous coat.
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Old 31st Jul 2020, 3:41 pm   #22
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Default Re: What causes this in polyurathane varnish?

Its the rustins stuff. I don't remember any problems with the previous cans. Infact I have some left in a small can, I might try and see if any different. Ive bought exactly the same product down to the barcode being the same.

I wonder if the weather as well isn't helping things too, but suspect not.

The varnish is this stuff: https://www.uktoolcentre.co.uk/produ...iABEgK0S_D_BwE

Though not from that retailer, and infact I think it was a different retailer to the first can...
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Old 31st Jul 2020, 3:45 pm   #23
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Default Re: What causes this in polyurathane varnish?

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Well, blast. I thinned the polyurethane with white spirit, got it nice and liquid like, no treacle. Applied with a brush as thinly as I could, and again I have a mess

I will have to try again and see if I can spread it thinner still
As I say in post 18, interaction with previous coat.
Possibly, but the previous coat was the one with the wrinkles, except sanded back. After sanding I left it a day before trying to coat it again.

If I have to be that precise with the time frames between coats I might well never finish this

If you're correct I suppose I will have very lightly sand this and build some more layers over a few days.

I try to be scientific with this, because Im very inexperienced when it comes to finishing. I take the attitude if its not right, I did something wrong, therefore go back and try something different.

EDIT: so if it is interaction between coats, any idea what an ideal interval might be? I take it I want it cured to a point, but perhaps not entirely cured such that the new coats spirits start to dissolve it?
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Old 31st Jul 2020, 3:55 pm   #24
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Default Re: What causes this in polyurathane varnish?

Might also be worth double checking that the varnish is suitable to thin with white spirit - not all are.
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Old 31st Jul 2020, 3:56 pm   #25
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Default Re: What causes this in polyurathane varnish?

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Might also be worth double checking that the varnish is suitable to thin with white spirit - not all are.
Definitely is with this one, says so on the can
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Old 31st Jul 2020, 4:31 pm   #26
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Default Re: What causes this in polyurathane varnish?

try it on another small piece of wood, not veneered
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Old 31st Jul 2020, 4:49 pm   #27
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Default Re: What causes this in polyurathane varnish?

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Well, blast. I thinned the polyurethane with white spirit, got it nice and liquid like, no treacle. Applied with a brush as thinly as I could, and again I have a mess

I will have to try again and see if I can spread it thinner still
As I say in post 18, interaction with previous coat.
Possibly, but the previous coat was the one with the wrinkles, except sanded back. After sanding I left it a day before trying to coat it again.

If I have to be that precise with the time frames between coats I might well never finish this

If you're correct I suppose I will have very lightly sand this and build some more layers over a few days.

I try to be scientific with this, because Im very inexperienced when it comes to finishing. I take the attitude if its not right, I did something wrong, therefore go back and try something different.

EDIT: so if it is interaction between coats, any idea what an ideal interval might be? I take it I want it cured to a point, but perhaps not entirely cured such that the new coats spirits start to dissolve it?
The extra white spirit you used could also have had a more 'aggressive' effect on the previous coat, softening it and causing it to ripple.

To get off subject slightly, I no longer use polyurethane varnishes having had too many problems in the past and never really got a great finish. I use the new generation of 'milky' finishes that go on like melted butter, dry quickly and self level without any sign of brush strokes or anything like that. The stuff is milky from the can but it dries perfectly clear. I've used it extensively in recent years. I realise it's no use to you now, but for future projects I'd definitely consider it. I use Wilko clear varnish. They do it in gloss, satin and matt and in stain colours too. You simply will not beat it to get a great, hassle free finish every time. I used it in this project: https://www.vintage-radio.net/forum/...d.php?t=130650
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Old 31st Jul 2020, 5:08 pm   #28
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Default Re: What causes this in polyurethane varnish?

Is that on top of the previous varnish? A repeat of others, I missed page two.
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Old 31st Jul 2020, 5:08 pm   #29
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Default Re: What causes this in polyurathane varnish?

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To get off subject slightly, I no longer use polyurethane varnishes having had too many problems in the past and never really got a great finish.
I'm the same and I think Adam's experiences are an illustration of why many people give up on brushed polyurethane. Personally I use sprayed acrylic for a high gloss finish and oils for a more natural but less protective result. However, I do admire Adam's determination and perseverence in the face of adversity!

Alan
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Old 31st Jul 2020, 5:24 pm   #30
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Default Re: What causes this in polyurethane varnish?

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Is that on top of the previous varnish? A repeat of others, I missed page two.
Previous varnish I have applied, I.E same stuff. It was plain veneer when I started which has been lightly stained, I have been building layers of poly and sanding between. I really thought I was reaching the finish line with this, I.E just one more top coat, a few weeks to ensure its cured then on to the buffing stage before assembling all my bits. But unfortunately events or my methods are now conspiring against me!
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Old 31st Jul 2020, 9:57 pm   #31
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Default Re: What causes this in polyurethane varnish?

Something I learned via a distance learning course many years ago. Paint doesn't dry, it cures. It's not simply about the solvent evaporating whatever solvent that may be. Someone has already said this. The oils in the paint oxidise.The same effect can be seen with cooking oil on pans, cookers and extractors.

I'm not sure whether polyurethane varnish works in the same way, but I'd expect it does. So recoating would need to be done either before the first coat has started to cure, or after it has cured well.
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Old 1st Aug 2020, 9:06 am   #32
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Default Re: What causes this in polyurethane varnish?

Odd that - I've never had a problem with that material, even when brushing on in spades. The one exception being a table with oak veneer, which never achieved a perfectly flat surface, even after repeated light sanding and further coats.
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Old 1st Aug 2020, 2:32 pm   #33
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Default Re: What causes this in polyurethane varnish?

Iíve just sanded and applied a new coat from an old tin that id previously not had issues with, just to see if the problem presents itself again. I applied it thicker than last time but not too thick. Should see the results by evening...
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Old 1st Aug 2020, 5:41 pm   #34
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Default Re: What causes this in polyurethane varnish?

Well, it isnít the varnish. Same result with the old can, which leaves environmental factors or layer interaction. Looking like later interaction is the problem. So I will have to now build a few coats over a period of days before I can correctly sand these blemishes out. This varnishing lark really is a pain in the backside!!!

I donít suppose thereís anything nice and easy that could be applied as a top coat over this now? Iím doomed to finish with poly now arenít I?
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Old 1st Aug 2020, 6:23 pm   #35
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Default Re: What causes this in polyurethane varnish?

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I don’t suppose there’s anything nice and easy that could be applied as a top coat
Nope, either wait for a few weeks and sand down or strip the lot and start again. No such thing as an easy finish, factories spend 10's of thousands on finishing plant (car plants probably millions), us mere mortals have to take time and patience.

No easy way out.
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Old 1st Aug 2020, 7:00 pm   #36
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Default Re: What causes this in polyurethane varnish?

So what would I be waiting for? For the layers of poly to be completely cured?

From what I understand you either apply layers in quick succession, say a day or so between. Or wait for absolute curing and then sand a little to help adhesion? My mistake therefore must have been to attempt something at an in between stage?

So I could either build more coats on top of the ďcrazedĒ and wrinkled ones, and through sanding smooth that out

OR

Surface sand the lot, forget about it for a month or so then revisit?

Does that sound correct?

I think taking it all off wonít be an option. Iíd need to re-apply veneer to the whole project. Itís very thin and the stain is a surface stain that is easily sanded through.

Appreciate all the advice guys, it is what it is, Iím not giving up!!!

Cheers
Adam
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Old 1st Aug 2020, 7:14 pm   #37
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Default Re: What causes this in polyurethane varnish?

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So what would I be waiting for? For the layers of poly to be completely cured?
Yes, just leave it for a month then sand it flat (wet and dry used wet is good on a fully cured finish, Halfords do a very good multi pack of loads of grades, end with the finest) and then a final thinned down coat.

It may well take more than a month, if it takes a thumbnail impression at all it's not ready.
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Old 1st Aug 2020, 8:58 pm   #38
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Default Re: What causes this in polyurethane varnish?

Ok, I have various pieces in stages. Is there any harm to sanding them before the 4-6 weeks I leave them?

I guess it gives me a well earned excuse to take a break from this project for my mental health
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Old 13th Sep 2020, 9:03 am   #39
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Default Re: What causes this in polyurethane varnish?

Just and update, sure enough merlinmaxwell was correct.

After waiting just over a month I have sanded the piece, re applied varnish and there are no wrinkles or abnormalities at all.

Now I just need to de-nib it
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Old 13th Sep 2020, 9:15 am   #40
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Default Re: What causes this in polyurethane varnish?

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Just and update, sure enough merlinmaxwell was correct.

After waiting just over a month I have sanded the piece, re applied varnish and there are no wrinkles or abnormalities at all.

Now I just need to de-nib it
Like I explained in post 18. Take my advice and don't use that stuff again, use the new milky varnishes as sold by Wilko. It's absolutely problem free and a perfect finish every time. Great news that you got there in the end though, well done.
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