An irritating problem

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Kittyhawk

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I have a finishing problem in the aircraftery that I can live with ok - its just the not knowing why that is driving me crazy.
All my models are built of NZ heart Rimu and the finishing is identical on all of them.
The bare wood is sanded to 400 grit and gets a coat of sanding sealer. After 24 hours the models get another light sand with 800 grit and then the first coat of Briwax teak oil. This is followed by a further 3 coats of teak oil at 24 hour intervals with a rub down with 0000 steel wool between coats.
Easy and straightforward except that coat No.2 of the teak oil takes up to 3 days to dry. All the other coats are touch dry in 4 - 6 hours, including the first teak oil coat. This has been going on since day one in the aeroplane business.
I can speed up the drying a bit of the second teak oil coat by using a UV lamp but it will still be 36 - 40 hours.
There is no logical explanation I can think of for this. Any ideas?
 

Inspector

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Just a guess. Could coat No.1 be reacting with the sanding sealer? What happens if you skip the sanding sealer and just start finishing with the oil?

Pete
 

Droogs

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The first coat soaks into the wood and crystalizes just below for most of it and just at the surface. This dries fairly quickly, the 2nd coat is actually sitting as a little lake on top of this and is trying to penetrate the semi sealed surface to bond chemically with the first coat. It sits there and is probably around 3 x thicker. The surface in touch with the air will start to cure first and as the air penetrates chemically change the as it passes through until reaching the already cured first layer. This takes time due to the thickness of the layer. the 3rd and subsequent coats when applied are much thinner due to the fact that the cured surface "grabs" and holds a much thinner layer when the 3rd is applied and this is able to cure more quickly as with following coats. This is all very layman terms but I hope it helps explain the process.
 

Sgian Dubh

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I'd say you're applying too many products and too many coats which will all be contributing to the slow drying. The teak oil, discontinued here in the UK, is its own base coat, so I can't see any good reason to apply a sanding sealer, which I'm guessing could be either a shellac or cellulose product, and then four coats of your teak oil: that's five coats including the sealer, at least two coats more than what the manufacturer recommends. The Rustins (Briwax) application guidance suggest you should apply two or three coats to prepped bare wood - no mention of using a sanding sealer. Teak oil is basically a wiping varnish which means its been formulated with less resin than a normal brush applied oil based varnish, but with a higher proportion of oil and white spirit, made that way so that's it's easy to apply, leave for a while and buff off.

I guess you're still able to buy Briwax teak oil in NZ, but if the product also becomes unavailable there as well as here, you could switch to their Danish oil, which is much the same product, but I suggest you try following the application guidance on some scrap wood whichever product you use to see if you eliminate the long drying time and still get the same look and protection you need. Slainte.
 

Kittyhawk

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Thanks for the replies.
I know Briwax says 2 - 3 coats on the tin but because of the complexity and delicacy of the shapes to which I am applying the oil I am using a 10mm artists sable brush to put it on with. So each coat is pretty thin and is why 4 coats are needed to get a decent finish. Briwax is made under licence here in NZ and my particular application method is to their advice.
The sanding sealer is a Feast-Watson product made in Australia and I don't know what the base is - its thinnable with Turps and there is a reason for using it on the models.
As in my original post, this is not a huge problem, its just curious that the protracted drying only occurs to the second of the four coats of oil.
 

niall Y

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I have noticed, with oil finishes , that subsequent coats dry a lot quicker. - Its that first coat that seems to take so long to dry. I have often wondered if there is some form of chemical reaction with each subsequent coat as it bonds with the previous one, that gives rise to this much faster drying
 

Kittyhawk

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I have noticed, with oil finishes , that subsequent coats dry a lot quicker. - Its that first coat that seems to take so long to dry. I have often wondered if there is some form of chemical reaction with each subsequent coat as it bonds with the previous one, that gives rise to this much faster drying

Yesterday was second teak oil coat on a model day.
This is the coat with the usual protracted drying. What I have been doing in the past was to apply it and after a few hours put the UV lamp on the model if the teak oil was not curing.
Yesterday I put the lamp on straight away and the coating dried within two hours. Not sure but I think the gloss was marginally reduced, but the third coat today is drying normally as per usual without any UV assist.
So this appears to solve the issue - the only problem now is that my antiquated Phillips sun lamp doesn't want to light up anymore. More expense.
 

Sachakins

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Have you tried drying outside in the sunlight, natural UV, may need a clear perspex/glass cold frame to protect from dust though?
 

Kittyhawk

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Have you tried drying outside in the sunlight, natural UV, may need a clear perspex/glass cold frame to protect from dust though?
Yes, sunlight would be the preferred option but we haven't seen that for weeks. This is the wettest most miserable winter I can ever remember. ☹️
 

Sachakins

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Yes, sunlight would be the preferred option but we haven't seen that for weeks. This is the wettest most miserable winter I can ever remember. ☹️
Sorry, just realised your location, NZ.
New UV lamp expense it is then 🤑
 

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