Tung oil finish curing rough? How, why & what to do?

[will1983]

I'm looking for others opinion on an issue I have come across this week.

I built 8 doors for some wardrobes out of beech and finished them with 4 coats of Liberon quick drying Tung oil. I sanded these to 240grit before applying the first oil coat and dry flatted the finish after coat no2 with 400grit, the final coat was applied and then wet flatted with 600 grit.

Initially the finish came out very smooth and I was quite happy with it so I went ahead and fitted the doors. However that was several weeks ago so the oil has fully cured now but the surface feels somewhat rough. Also several dry spots have appeared where the pores in the timber are not fully filled, when I fitted them no dry spots were evident.

It looks like either the oil has sunk deeper into the timber and raised some of the grain or that some of the excess oil has been ejected from the pores as it has cured and then dried crusty on the surface.

Has anyone else experienced something similar? I presume that I need to flat the finish again and apply yet another coat? I have a lot of work invested in these so would like to get them looking as close to perfect as I can.

 

[profchris]

This is a standard problem when finishing musical instruments. The film finish shrinks as it cures, and shrinks right back into the pores. You can keep going until all the pores are filled, but that will be a few cycles of waiting weeks to see how much the finish shrinks and then adding more coats, repeat ... The alternative is some positive pore filling, but that doesn't sound right for these doors.

I get the roughness after my first coats of shellac have cured, but I sand after the initial couple of coats have cured before adding more finish - sounds like you have the same problem, and it shouldn't come back so long as you don't sand through your cured finish anywhere!

 

[will1983]

Thanks for the reply Chris. My gut feeling is that it has indeed shrunk back into the timber.

So to confirm, my remedial actions should be to lightly key the finish and apply further coats, do you think 2 will be enough?

 

[thetyreman]

I used pure tung oil on a TV cabinet I made, the oil definitely takes ages to fully cure, I'd say at least 2-3 weeks, I took the cabinet inside and oil was still coming out every day for about 3-4 days, it took about 7 days before the oil stopped oosing out of the pores, that was with 5 layers of the stuff, so admittedly probably too much, I think 3-4 layers seems right, with the first 2 layers thinned out, I didn't really notice any roughness, because I wiped it away before it could dry.

 

[Silly_Billy]

I used pure tung oil on a TV cabinet I made, the oil definitely takes ages to fully cure, I'd say at least 2-3 weeks

Is this about right for tung oil? I applied the last coat of pure tung oil to a chopping board a week ago. It looks great, but feels oily to touch. Do I simply need to leave it alone for 2-3 weeks?

 

[Eric The Viking]

I'd say so: the DC has a favourite small kitchen knife, and occasionally I do that with Tung oil. It takes bloomin' ages to go off, but then lasts for a year or more.

I did a neighbour's hatchet handle (hickory) last night. It was dry to the touch this morning (that's neat oil wiped on thinly after scrubbing it clean-ish with 0000 grade wire wool). The bits where the wood was properly clean look good, but it was covered in crud, so will never look brill as there's too much dirt in the grain (and I didn't feel that charitable!). The Tung oil has definitely not fully cured though - still smells strongly, which means something is still evaporating.

 

[ED65]

I applied the last coat of pure tung oil to a chopping board a week ago. It looks great, but feels oily to touch.

How scrupulously did you wipe off after application?

The gold standard for oiling is for the wood surface to be basically dry when you're done. Not dry in the sense of cured of course, but there shouldn't be any detectable oily feel, that's a sign too much was left on the surface.

 

[Silly_Billy]

Thanks ED65. I did wipe off the oil. However, perhaps I wasn’t scrupulous enough!

I’ve been experimenting with using different oils and am finding them all a bit different.

 

[ED65]

I’ve been experimenting with using different oils and am finding them all a bit different.

I'd expect so! Most wood-finishing products that are called an oil on the label, best example probably being Danish oil, are some form of blended finish, an oil mixed with some varnish (a lot or a little) and then generally diluted with quite a bit of additional white spirit. Because they vary so much no two are quite alike even if named the same, and collectively they have a completely different character to BLO or pure tung oil, which are in their turn different enough from each other that they're often not applied in the same way.

 

[custard]

In some commercial workshops they wipe off the first few coats of oil with a rag that's been very lightly dampened with thinners. That's to speed up the process and ensure the intermediate coats are in the wood rather than on the wood. Only when the workpiece isn't soaking up any more oil do you apply the final coat or two...and then park the job out of the way and let it dry for a couple of weeks before burnishing like mad.

 

[thetyreman]

In some commercial workshops they wipe off the first few coats of oil with a rag that's been very lightly dampened with thinners. That's to speed up the process and ensure the intermediate coats are in the wood rather than on the wood. Only when the workpiece isn't soaking up any more oil do you apply the final coat or two...and then park the job out of the way and let it dry for a couple of weeks before burnishing like mad.

what do you recommend for the final burnishing? for example I've read that steel wool can be used, and I've also read that you can use a lint free cloth, from the sounds of it, I've not been leaving it long enough, it seems like there's very little information on the actual burnishing process.

 

[custard]

Burnish with a coarse cloth like hessian or sacking (the stuff you get 56lbs of spuds in is ideal). Don't use an abrasive like steel wool, or on the edges and corners you'll be through to the bare wood in no time. Don't use towelling, sooner or later one of the fibre loops will snag and tear a splinter out of the workpiece.

Burnishing is why most people stopped using rubbed oil finishes. Even with a polishing bonnet on a power tool it's just too much work to make it practical. For truly exceptional boards of Walnut or Rosewood, with the kind of extraordinary figuring that just pulls you into the surface, then maybe. But for everything else it's just not worth the effort when solutions like Osmo give you 90% of the benefit for 10% of the work.