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Old 30th Jul 2020, 1:15 pm   #1
PsychMan
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Default What causes this in polyurethane varnish?

Ive been working on finishing a motor board for a deck, and cabinet I am building.

Im at the stage where Im trying to do my final coats before 2000 grit sandpaper and final buffing.

What I am finding is I apply the coat, go back the next day, and around the edges I have what I would call "crazing", and Im not sure why its happening or how to prevent it. See picture

Ive noticed it happening a lot more since I started a new tin of varnish, which is therefore a bit less viscous than the end of the last can. So this could be a factor.

Any tips on how to prevent this would be much appreciated!

I've spent what feels like months on this. The motor board is salvaged from an old radiogram, and has been sanded, re-veneered and stained, and several coats of poly applied and wet sanded sanded to fill the grain. Ive built a back and sides from MDF and applied the same veneer.

Im new to all this cabinet melark but want this to turn out nice, so Ive hit the "reset" button several times and re-veneered everything due to mistakes. Patience, like the veneer, is now very thin!


Cheers
Adam
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Old 30th Jul 2020, 2:50 pm   #2
unitelex
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Default Re: What causes this in polyurathane varnish?

Difficult to see from the photo but could it be due to shinkage as the solvent evaporates?

If there was more solvent in the latest tin then maybe there is more material evaporating therefore more shrinkage?

Best Regards
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Old 30th Jul 2020, 3:00 pm   #3
BRASSBITS
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Default Re: What causes this in polyurathane varnish?

are both varnish's the same brand and type?
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Old 30th Jul 2020, 3:16 pm   #4
PsychMan
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Default Re: What causes this in polyurathane varnish?

The bits that reflect the light, the wrinkly bits are the issue. They have texture which can be felt also. Its always near the edges too, and never in the middle of a surface.

Its exactly the same brand and type, same barcode etc and both bought in a short space of time from the same source.

The solvent theory does sound plausible, though how to prevent it is a tough one. I do give it a stir every time I use it. The new batch being thinner I assumed was due it being new, I.E the old one got thicker at the end.

One more point that could be relevant, I have been leaving the brushes in white spirit between use, and wiping dry with a rag before using it. Perhaps Im adding more spirit through the residue...
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Old 30th Jul 2020, 3:25 pm   #5
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Default Re: What causes this in polyurathane varnish?

Actually its not just the edges.

The attached picture gives a clearer picture of what is happening.

At the time of applying, the coat looks smooth and fine, with no marks like this
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Old 30th Jul 2020, 3:30 pm   #6
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Default Re: What causes this in polyurathane varnish?

Any chance it could be due to putting it on too thick ?
Coated too thickly, it could easily take much more than 24hrs to fully cure.
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Old 30th Jul 2020, 3:31 pm   #7
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Default Re: What causes this in polyurathane varnish?

A combination of applying coats too thickly and insufficient curing time between coats I suspect.

Alan
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Old 30th Jul 2020, 3:37 pm   #8
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Default Re: What causes this in polyurathane varnish?

Could be a reaction to traces of something incompatible maybe traces of an original coat left in the wood.

I didn't know polyurethane was too bad. Cellulose is very touchy.

David
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Old 30th Jul 2020, 3:40 pm   #9
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Default Re: What causes this in polyurathane varnish?

Contamination usually causes bubbling or crazing rather than wrinkling.

Alan
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Old 30th Jul 2020, 3:45 pm   #10
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Default Re: What causes this in polyurathane varnish?

Interesting thoughts. The coats have been thick, admittedly, On the bits ive had good results with I have applied thickly and its left a lovely smooth finish. I've been giving them a long time to cure before recoating. Before doing this final coat, it must have been about 2 weeks , and I did sand it quite heavily with wet 600 grit paper.

I wonder if environmental issues could be a factor. My shed is quite warm, and potentially a bit humid in this weather. Its insulated quite well, but later in the day the sun seems to have baked through it enough to heat the room up.

Perhaps some moisture has been left from my wet sanding too, I wiped it dry but maybe that wasn't enough.

Im going to do an experiment this time. Im wet sanding it as before, but will leave the piece overnight to make sure its free of moisture. I'll also run my dehumidifier on full overnight, and when I do the next coat, apply it in the morning or evening when the weather is cooler.
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Old 30th Jul 2020, 4:43 pm   #11
Craig Sawyers
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Default Re: What causes this in polyurathane varnish?

Humidity is my guess.

I'm a bit of a traditional luddite and would have used French polish. Takes some practice to get it right, but the beauty is if it all goes wrong, you wipe it off with meths and start again. This is relatively easy on small areas like a record deck or radio cabinet. The really skilled specialist polishers are needed for dining table size jobs.

With solvent varnishes, when it goes wrong it is a whole load a grief with sanding it off with potential damage to the veneer.

Sorry it has gone pear shaped - bit late with the French polish method I'm afraid.

Craig
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Old 30th Jul 2020, 5:05 pm   #12
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Default Re: What causes this in polyurathane varnish?

I reckon the surface of the varnish has dried too quickly, shrinking as it dries, over the wet varnish underneath. It needs to be applied more thinly.

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Old 30th Jul 2020, 5:49 pm   #13
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Default Re: What causes this in polyurathane varnish?

Coats much much too thick. To get a good finish the last coat can be thinned down to disperse brush marks.
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Old 30th Jul 2020, 6:01 pm   #14
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Default Re: What causes this in polyurathane varnish?

...addition
Air curing finishes create a skin on top by oxidising the top layer, once this happens it is soluble in the solvents used in the finish. This creates a bag of wet varnish and causes these problems. A thin coat is about the thickness of the "bag" lets the oxidation happen through the whole layer.

Two pack finishes react with themselves through the layer, no problem. Similarly with solvent drying ones (like cellulose lacquer) let the solvent out by partially dissolving the (now semi dry) top coat.

Dip "painting" tends to level all the variables out and works well, that is if you have enough to make a dip bath! As an aside (nearly) take a look at spin coating, despite the surface spinning at a low velocity at the centre and quickly at the perimeter an even coat is formed, the maths is involved, the result rather clever.
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Old 30th Jul 2020, 7:40 pm   #15
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Default Re: What causes this in polyurathane varnish?

I'd agree that judging by the depth of the wrinkles, the varnish needs to be applied far more thinly, and preferably not with a brush.

Oil base varnishes (including polyurethane) take many hours to dry, and they don't just 'dry' - they cure by a chemical reaction that takes place after the thinner has evaporated. The reaction is brought about by the absorption of oxygen. (The same applies to finishes such as Danish Oil). The thicker the coat, the slower it will dry. My guess is that your ripples have occurred because the outer surface of the varnish has dried, but as the varnish coat was too thick, the underside of the 'skin' has dried more slowly.

If applying polyurethane for what amounts to cabinet finishing, (as opposed to say a sealing a floor), straight from the tin it's probably a bit too 'treacly'. I'd dilute it about 20% with whatever is suggested on the tin - EG white spirit or whatever. It's easy to load a brush with too much varnish and I think you'd probably do better to use a foam (or 'sponge') brush which will ensure that you can only pick up enough varnish to lay on a thin coat.

I'm not recommending these particularly, but they're an example of what I mean:

https://www.amazon.co.uk/dp/B088K84L...NsaWNrPXRydWU=

If you prefer to use a brush, then a synthetic bristle one would be better for two reasons. Firstly, you can't 'load' the brush as much as a normal bristle brush, and secondly, the filaments are 'feathered' so unlike a normal bristle paintbrush brush they don't leave brush marks, which reduces the temptation to continue to brush the varnish. Something like these, for example:

https://www.screwfix.com/p/harris-tr...E&gclsrc=aw.ds

To get a successful finish with any varnish you can't apply it like you would paint, when for example, painting a door or skirting board, brushing the paint back and forth. When applying varnish to a flat surface, you should do it in strips that don't overlap too much, starting at the centre of each strip, brushing slowly towards the end, then again at the centre of that strip, brushing to the other end - never back and forth, and that should be it. If you're not quite happy with it, you can 'feather' the varnish - use the brush upright, and slowly pull it along literally like a feather, with a featherlight touch. (If you apply water based varnishes of any kind, only every use a synthetic brush as water will cause natural bristle brushes to splay into little bunches).

If you apply varnish as you would paint, you'll apply too thick a layer and you'll introduce air bubbles. Likewise, when you charge your brush with varnish, just dip it in the tin and allow the excess to drip off - don't do as we tend to do with paint and scrape any excess off on the lip of he tin, or that will introduce air. Also, never shake a tin of polyurethane or that will create air bubbles which will take forever to come to the surface. Only ever give it a gentle stir.

It's a long time such I've use polyurethane varnish but when I did, I used to apply it with a 'rub' rather like French polish, drawing it along the wood in thin coats, and applying four or five coats, allow each coat to dry thoroughly. The only radio related thing I have which was finished with polyurethane is the mahogany base of a Morse key which I made from one that had a smashed Bakelite base and was in bits. Obviously not a challenging project compared to yours, but it's an example of a rubbed finish. The first pic is the cleaned up components of the key, the second pic is the key assembled on the base.

I'm not setting myself out as an expert - I'm just a hobbyist like the rest of us and I've had as many failures as anyone. I'm just basing these remarks on what I've learnt over the years. Winston Churchill once said 'Success is what comes of going from one failure to another with no loss of enthusiasm'.

I hope this helps a bit.

Every good wish with it.
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Old 30th Jul 2020, 8:24 pm   #16
PsychMan
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Default Re: What causes this in polyurathane varnish?

Thanks all, especially merlinmaxwell and David for such a detailed replies. So consensus does seem im coating too thick. Perhaps I got away with it with good results at the end of the can when it was thicker and the curing process had already started to a point.

So if I thin it with white spirit and spread it thinly I shouldnít have problems with brush marks? I will certainly try it tomorrow. I have at least been careful to apply it with minimal brush strokes, and minimise any spreading and overlap, so Iíve been doing some of it correctly. Perhaps thinning is the key then, Iíll report back how it goes. The reason I chose this varnish in the end is it does go on much thicker than aerosol lacquer and over several coats will fill the grain very well. The amber hue it applies also worked perfectly with the stain and gave me the look I was after

French polish does appeal to me, but Iíd need to invest a bit for that and spend some time learning. I do intend to do that, but for now I just want to finish this project. Itís been running so long I am losing interest, but Iíve already spent so much time on the cabinet, not to mention a complete tear down and renovation of the deck (Garrard RC121 mk2) that thereís no way I could abandon this now
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Old 30th Jul 2020, 9:14 pm   #17
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Default Re: What causes this in polyurathane varnish?

A couple of tips;

1. Consider the use of a foam paint roller instead of a brush for large areas.

2. Polyurethane lacquer is available as an aerosol can. Easier to control layers.
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Old 30th Jul 2020, 9:27 pm   #18
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Default Re: What causes this in polyurathane varnish?

I remember Ronseal (?) used to talk about there being a critical time window between coats. Too soon and the new coat stops the one below from drying and causes interaction and surface rippling. Too much time lapse and the new coat starts to soften the old coat, again causing surface problems. Something like that. I think eight to 24 hours - something like that, is the recommendation. Don't quote me on it, but there is an optimum recommended window of time between coats. Maybe that's your problem. Plus coats being too thick, that would exacerbate the previously described scenario.
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Old 31st Jul 2020, 3:07 pm   #19
PsychMan
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Default Re: What causes this in polyurathane varnish?

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
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Old 31st Jul 2020, 3:26 pm   #20
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Default Re: What causes this in polyurathane varnish?

I wonder if it's the varnish itself, is this a DIY shop version? I used some 50 year old 'yacht varnish' applying it thick and had no problems.
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