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Old 3rd Apr 2020, 1:30 pm   #1
PsychMan
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Default Polyurethane Sanding Question

Hi Folks,

Thought id try my hand at gloss finishing a little box Im making to house a project its plywood with walnut veneer that has been stained. I bought several grades of sandpaper (100,200,400,600 etc etc) amd Im using rustins gloss poly varnish.

It advises 6-8 hours between coats depending on conditions. As this is in my cold workshop ive waited 24 hours before sanding the coats. It didn't feel at all tacky, But Im finding the sandpaper is getting gummed up and its sanding to a "gluey" residue.

Trying to sand through this has resulted in me sanding through some of the stain on the veneer. In total its had 4 coats now, and after each coats it was roughed up with 400 grade sandpaper. Each "between coat" sanding did result in some white residue, I thought this was normal and I was able to wipe off most of it, and the rest became hidden with the next coat.

But when trying to finish it off and buff up, im left with gluey mess and will likely need to start the face of this box again and re-veneer it now.

So Im wondering, what am I doing wrong?

My instinct is telling me 24 hours in a cold workshop has been insufficient and perhaps I should apply and leave it 24 hours indoors??

Any thoughts much appreciated. This is actually a test piece for a record changer plinth I'm building. I wanted something I cared less about to try and work out the method for doing a gloss finish - and good thing I did test it first!

Adam
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Old 3rd Apr 2020, 1:34 pm   #2
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Default Re: Polyeurthane Sanding Question

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My instinct is telling me 24 hours in a cold workshop has been insufficient and perhaps I should apply and leave it 24 hours indoors??
You are right, let most of the smell go away in the workshop (an hour or two) then indoors for a day or two. Sticking to the sandpaper can be reduced or even eliminated by using wet and dry wet.
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Old 3rd Apr 2020, 2:33 pm   #3
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Default Re: Polyeurthane Sanding Question

Thanks. I suppose its been compounded by several layers that have probably not entirely cured.

Ive found actually even wood glue used on veneer for example, after 24 hours still smells very gluey, and Ive took it into the house overnight to completely dry it. Based on that I should known better I suppose!

Ive used the power sander, taken all the veneer off the face of the box and am now reclamping a fresh piece to start again
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Old 3rd Apr 2020, 3:49 pm   #4
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Default Re: Polyeurthane Sanding Question

Oil based polyurethanes can take something like four weeks to fully cure depending on the thickness of the coat and the ambient temperature. For the less durable water based types it might not take quite as long. Achieving a really smooth high gloss finish can be a long haul. When doing this it's best to apply thin coats and build the finish gradually. Much patience required.

Alan
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Old 3rd Apr 2020, 4:17 pm   #5
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Default Re: Polyeurthane Sanding Question

The last varnishing job I did was the "meter clock", got some spray "Simoniz High gloss protection from oil and petrol" from Halfords, I thought I made a mistake when I tried it, stank to high heaven took a bit longer than other acrylics took to dry. Fantastically hard finish and very glossy. This https://www.saveanddrive.co.uk/produ...-500ml-simp22d
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Old 3rd Apr 2020, 5:06 pm   #6
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Default Re: Polyeurthane Sanding Question

Blimey!! No wonder im having problems.

I don't need perfection on this, just enough experience to put more time in when I finish my record changer plinth.

With hindsight my coats were a bit thick too. Thin coats and 48 hours inside and ill see what difference that makes
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Old 3rd Apr 2020, 5:37 pm   #7
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Default Re: Polyeurthane Sanding Question

Sprayed acrylics have some distinct advantages because of the much shorter curing times (days rather than weeks). This is partly because each layer has to be very thin to avoid runs and partly due to the fast evaporation of the solvents. However, it's more dificult to arrive at the depth of gloss that can be achieved, albeit laboriously, with polyeurethanes. Personally I wouldn't use polyurethanes these days. Probably just laziness on my part although it's worth remembering in the context of this forum that most 'woodies' were originally spray lacquered at the factory.

Alan
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Old 3rd Apr 2020, 7:31 pm   #8
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Default Re: Polyeurthane Sanding Question

Something to bear in mind. Paint and lacquer don't dry, they cure. The solvent thins it enough for application especially when spraying, and this can mean that it's fairly quickly 'dry'. But it can still take a while to cure enough to be hard. Take a conventional gloss paint. It's oil based and essentially the oil has to 'cure' rather than dry. This is essentially an oxidisation process (as explained to me). Cooking oil can do the same thing, hence if it's left it gets very sticky and eventually just like varnish.

If you think about it, if it were simply a matter of the solvent evaporating, then re-applying some solvent would be able to simply wash off the paint, which doesn't happen. Especially important if you think of water based paints, like emulsion for your walls, and these days almost all factory car paint.
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Old 4th Apr 2020, 1:07 pm   #9
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Default Re: Polyeurthane Sanding Question

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Sprayed acrylics have some distinct advantages because of the much shorter curing times (days rather than weeks). This is partly because each layer has to be very thin to avoid runs and partly due to the fast evaporation of the solvents. However, it's more difficult to arrive at the depth of gloss that can be achieved, albeit laboriously, with polyurethanes. Personally I wouldn't use polyurethanes these days. Probably just laziness on my part although it's worth remembering in the context of this forum that most 'woodies' were originally spray lacquered at the factory.

Alan
I'd agree with Alan.

Originally, radio cabinets were invariably finished with cellulose lacquer, but as with cars, in these safety conscious days, acrylic lacquer has become the norm. As a woodturner, like many other turners, I used to use friction polish until a year ago, which is basically shellac (French polish), which is applied with a cloth on the lathe and dries in a minute or so. A clean cloth is then pressed against the item, with the friction generating heat, melting the polish to provide a high sheen.

Likewise, until maybe a year ago when I made little comb-jointed wooden boxes for home-brew test gear, etc. I used shellac sanding sealer, followed by a few coats of French polish. However, I now exclusively use high build gloss car lacquer in 500mL 'rattle cans'. A 500 mL of Tetrosyl Trade Spray Paint Clear Lacquer Acrylic Aerosol 500m is typically £5.99 post free on ebay. EG:

https://www.ebay.co.uk/itm/333114873765

It needs little skill to apply and dries quickly – I usually leave a couple of hours between coats to let it harden off. If the finish is too glossy, it can be toned down using 0000 grade wire wool. if it needs 'de-nibbing' between coats, I use 400g wet & dry paper between coats (used dry). If you need to lubricate the paper, it's better to use a sprinkle of talcum power rather than water. I don't have a picture of a radio on which I've used it, but attached is a pic of a bowl I recently turned ('spalted' sycamore), on which I toned down the gloss Tetrosyl with wire wool then polished with paste-wax. The second pic is four mahogany pattresses which I recently turned and sprayed with clear lacquer, left glossy. (I added those pics only to illustrate the finish that the lacquer gives - not to go off topic.)

U-Pol Power Can Clear Coat Aerosol 500ml is also excellent and sets really hard but it isn’t acrylic. It’s probably the closest you’d get to the original lacquer used on radio cabinets, but must really be used with caution outdoors on a fine day as it’s flammable, gives off hazardous fumes and must be used in accordance with the safety data sheet:

https://www.amazon.co.uk/UPol-Power-.../dp/B009G1AU3Q

https://www.u-pol.com/uk/en-uk/produ...t#.XohpfohKgdU

Don’t overlook Danish oil, which needs no expertise to apply and often been used by forum members with excellent results. Danish oil is an oil-varnish blend, using either polymerised linseed oil or tung oil, derived from the nut of the tung tree. The formula differs from one make to another, but is typically one third varnish and the rest is oil. As Duncan said in post #8 above, varnish doesn’t simply ‘dry’ - it undergoes a chemical change to polymerizes into a solid form when it reacts with oxygen in the atmosphere. It provides a hard-wearing satin finish, which I think is a bit more pleasing on the eye in a modern setting than the high gloss of yesteryear. It doesn’t have an objectionable odour so can be applied indoors and left in a warm dry room to harden off for two or three days between coats.

The third pic below is a Ferranti 045 'woodie' radio which my nephew fished out a skip and asked me to restore for him. It had been in the rain and was in a very poor state with crazed and crumbling lacquer. I stripped the old lacquer off, had to partly re-veneer the front, and finished the cabinet with three coats of Danish oil. He was pleased with the result, and so was I. I'm sure that other forum members can give better examples of the results obtained by Danish oil.

Hope that helps a bit.

Every success in your endeavours.
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Old 4th Apr 2020, 3:42 pm   #10
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Default Re: Polyeurthane Sanding Question

Interesting tip about using talc. I have used it as a lubricant when threading cables into conduits but using it with abrasive paper is a new one on me. I have often used W&D with water to rub down cellulose spray paint and oil paint, in the latter case leaving several days between coats to ensure thorough drying, but the paper does need regular cleaning: I use a fine brass wire brush, but it never gets it all off. Must try talc next time.
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Old 4th Apr 2020, 3:50 pm   #11
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Default Re: Polyeurthane Sanding Question

We also used talc for cables, mil spec. ones. The mil spec. talc was horribly expensive, we used Boots finest. Lovely smell too.
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Old 4th Apr 2020, 10:22 pm   #12
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Default Re: Polyeurthane Sanding Question

Many years ago I refinished a mahogany coffee table and TV cabinet using polyurethane varnish applied by brush, can't remember how many coat's but would have been lightly rubbed down between applications. I let it dry/cure (not sure how long) then flatter it back with wet & dry paper used wet gradually using finer grades eventually using cutting compounds finishing with a wax.
Came up like glass and was very durable although looking back it was an awful lot of work.

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Old 5th Apr 2020, 4:48 pm   #13
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Default Re: Polyeurthane Sanding Question

Thanks to all for the comments.

Particularly David, whose posts I always enjoy reading! That tetrosyl lacquer looks very good, and sounds like it might give me the results I want with much less effort. I will continue with the poly for this small box, if only to get some experience. But I have another box to do next so perhaps will get a can of that to try

The only other issue I have I suppose is the veneer is not grain filled. I was hoping to fill it by building up more varnish coats. I’d read about sanding with danish oil to great an oil slurry to fill the grain, but it’s thin veneer so that’s a bit risky. I also managed to stain the main piece I’m working on which also limits options. I did experiment with a grain filling paste but found the dye contained in it was too dark for my liking and the paste cured way too quickly
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Old 5th Apr 2020, 5:23 pm   #14
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Default Re: Polyeurthane Sanding Question

It's as well to use sanding sealer which will fill the grain, prior to using varnishing. Sanding sealer is basically lacquer with zinc stearate added, which is a 'soapy' substance which fills the grain. The zinc stearate settles to the bottom of the container, so needs a good shake before application. You can apply it with a brush or cloth.

It comes in both cellulose and shellac - the cellulose type dries quicker, but the shellac dries in a matter of minutes anyway, then you can lightly sand it before applying whatever varnish you wish to use. As you say, modern veneer is very thin - usually just 0.6mm so you need to be cautious about sanding. It's become rather expensive, especially in small quantities such as 250 mL and if ordered by post.

Chestnut and Liberon brands are well regarded. EG:

https://www.ebay.co.uk/itm/Chestnut-...Sn60fJ47ATESYA

Hope that helps.
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Old 6th Apr 2020, 9:51 am   #15
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Default Re: Polyeurthane Sanding Question

Sounds great, Ill order some and give it a try.

Would something like this have been used for radio cabinets in the past?
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Old 6th Apr 2020, 10:58 am   #16
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Default Re: Polyeurthane Sanding Question

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Sounds great, Ill order some and give it a try.

Would something like this have been used for radio cabinets in the past?
Quite possibly. The veneers used were often quite low grade and were 'sexed up' by the application of 'toner' before being sprayed with lacquer. Sometimes, two or more shades of toner were used to simulate different veneers. The 1930s Philips Superinductance 634A 'Ovaltiny' radio is a prime example. The plain veneer is uninspiring, but in good original condition, with two shades of toner, looks attractive. Two bands of darker toner give the impression that two types of veneer were used, but that isn't so.

As often as not, the lacquer is flaking off, which is to be expected after eight decades, and arguably, the cabinet should be left alone (conserved) because proper restoration is beyond the scope of most of us. However, it's easy to see the temptation to want to strip the cabinet and slap on a coat of modern water based varnish. But in stripping the old lacquer, off will come the two shades of toner, because toner isn't like stain - it doesn't penetrate the veneer. The authenticity of many of these sets has been destroyed by DIY attempts, evidenced by the lack of the two darker bands after 're-varnishing'.

Below is a picture of what the set should look like.

I suspect that it's been restored professionally to a high standard. It looks too 'new' to be otherwise.

The second picture shows a typical example of a cabinet flaking lacquer, where it's evident that it isn't two contrasting veneers, but just a band of toner. I can understand the desire to want to strip and re-varnish the set, but from my perspective, the older it is, and if it's considered not just old, but a classic set with some unique features, conservation & preservation become important considerations.

I don't have any woodies that are of any value, but which I find attractive and have enjoyed restoring.

Over the years, going back to the early 1960s, I've had no hesitation in stripping off the old lacquer and have used various techniques to re-varnish them, from Ronseal to French polish, Danish oil and aerosol sprays. My yardstick has always been to try to attain the best possible DIY standard. I don't kid myself that I can aspire to professional results as I don't have the equipment or expertise to attain such a standard. What I lack of skills, as a hobbyist, I do have a commodity which professionals don't have - time! They do it for a living - I do it for enjoyment, and if I don't do it right, I can do it again.

Hope that's of interest.
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Old 10th Apr 2020, 8:15 am   #17
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Default Re: Polyurethane Sanding Question

Hi David is a ‘ Toner” the same as “ Stainer”? Thank you.
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Old 10th Apr 2020, 9:08 am   #18
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Default Re: Polyurethane Sanding Question

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Hi David is a ‘ Toner” the same as “ Stainer”? Thank you.
John
No - stain, whether water based or spirit based, will penetrate into the wood, and when it's in, it's in. The same applies to oil based finishes such as Danish oil, tung oil, teak oil etc.

Toner (as with lacquer) 'sits' on the wood, so if sanded back (judiciously!) will be removed, as will be the lacquer. That's why so many 'Ovaltiny' radios have been trashed when the two toner bands have been stripped off with the lacquer. If anyone attempted to replace the darker bands with stain rather than toner, even if masked off, the stain would creep along the grain of the veneer beneath the masking tape, whereas toner wouldn't.

The best way to sum it up is that with stain, the colour is in the wood - with toner, the colour is in the toner.

Sometimes, toner can be a hindrance on items which are handled a lot such as guitars for example, because if the lacquer is scratched or worn away in use, if toner is under the lacquer, that too will be removed.

Hope that helps.
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Old 10th Apr 2020, 11:30 am   #19
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Default Re: Polyurethane Sanding Question

Thank you yes that is very helpful.
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Old 11th Apr 2020, 5:38 pm   #20
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Default Re: Polyurethane Sanding Question

I think I have “toner” here, though it’s labelled as “dye”. I say that because I have found with sanding it is removed, much to my annoyance!!!

I think I will ditch the poly and go with the lacquer you recommended me David. Here’s why:

After re-veneering the face of the box, I applied the dye, and then a thinner layer of polyeurathane. I left this indoors for a few days and then for several hours in the sun. At thicker points of the coat you could push a thumb nail into it, indicated it still hadn’t entirely cured. I tried wet sanding it with grade 400, the water did help actually - but because the coat was thin, In places I managed to sand away the dye , the polyeurathane however managed to penetrate beneath where the dye did, result being the lighter areas won’t take dye now. ******. Time to veneer again for the third time and get some lacquer that is easier to work with!

Anyway, all a learning experience and exactly why I decided to attempt this on projects I’m not so concerned about!

The sanding sealer has now arrived thankfully, any tips on using this with the wood dye? Other than applying it afterwards? I imagine I might have the same
Problem with this dye if I’m not careful - or use a different product suppose..
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