question about wet sanding weird marks

[reck123]

so i have some wood here a combination of oak walnut and mahogny. i sanded the wood right up to 260. the applied 3 coats of oil based polyurethane. using a little steel wool inbetween coats and letting each coat dry overnight and the final 48 hours. finish was feeling ok at this stage so i decided to try a little wet sanding. i used some soapy water and started with 600 grit wiped down the surface and repeated with 1000. wiped down again and the piece is now feeling significantly smoother. but under close light inspection you can see marks on the wood that look like the varnish but a bit blotchey. it doesnt feel like i cut right through the varnish. what could be the reason and any solution? should i progress more with the grits in wet sanding. any help would be great!!!

I have attached a picture here

 

[CHJ]

Maybe you have not waited long enough for the finish to cure and harden to full depth, this could result in just removing the thinner cured 'surface skin' in patches.

I personally would steer clear of using wire wool near Oak.

 

[Droogs]

To me, Looks like you didn't wait long enough before sanding and have basically pulled the semi cured top coat away from the still uncured varnish underneath. you need to remember that although you get a fairly quick crust forming on the top due to exposure to air, it takes a few days for the stuff underneath to vent off and cure. i generally leave two days between coats if I can. Or you can speed things up by sitting the piece in front of an infra red heater, cuts the time in about half i've found.

hth

 

[ED65]

48 hours wouldn't generally be a long enough wait for oil-based varnish to harden enough to be sanded. You can get away with it if the drying conditions are warm and dry but it's best to wait longer, at least a week although many pros wait a fortnight to a month. These longer 'drying' times are especially important if you've applied more than a few coats.

It's hard to tell from the one photo if the shorter cure time may be part of the cause but I think the main thing is that you haven't cut back to the bottom of texture in the varnish, indicated by the shiny spots. Flat and matt areas are sanded, the shiny areas haven't been hit by the abrasive yet.

Starting with 600 may have been a contributing factor, although it depends quite a bit on whether you mean 600 or P600! But if the varnish application isn't quite smooth starting a bit coarser than 600 can be useful, e.g. with 400 grit which is a little finer than P600. And only once flat do you move up to the really fine grits, 600 and beyond (P1200 and up), to begin to polish the surface.

Anyway regardless of which you began sanding with further work with that grit may be what's needed. Sand until the surface is nearly completely flat, indicated by it being almost uniformly matt. Small shiny spots in recessed grain are likely going to be left and it's all right to leave them at this stage.

 

[Flynnwood]

With any oil based coating, it is best to wait seven days for all the solvents to evaporate/cure.

And as Chas said, oak and wire wool are not a good combination.

 

[reck123]

Thank you very much all for taking the time and giving me some good feedback. I used p600. Guess i jumped the gun here and wet sanded a little too soon. I have two other boards of exactly the same makeup tat i havn't damaged yet. it has now been roughly 72 hours since their last coat so guess i will give them a few more days before i sand those.

with the problem board,would you suggest sanding the finish off completly and starting over or re wet sanding in a few days to take off the other patches.

I have used steel wool inbetween the layers of varnish of all of the baords. it has made them a bit smoother and cant see any real damage in the oak. do you think i am still fine to wet sand in a few days on the other boards even though i used steel wool in the process?

Thanks for all the useful information so far guys.

 

[CHJ]

The problem with steel wool in proximity to Oak is that if any particles remain in contact with the Oak, sooner or later you may get brown/black staining if the Tannins in the Oak react with it.

 

[Phil Pascoe]

You can use steel wool on oak especially after it's been polished in some way many times without a problem - but when it's caught you out just once you won't do it again.

 

[Droogs]

my bad. only took in polyurethine and thought waterbased for my reply. Would leave about a week for oil based as already stated by Ed65. me head's a little fuzzy as getting over the flu and full of medication

 

[ColeyS1]

To be honest it looks like just lumpy varnish. Rub down with 320 between coats until it's smooth. Brush it on thinly, maybe thin it a little to make applying it easier.
I'm exactly the same using polyurethane. I put it on too thick and could never get a finish I was happy with. A part used tin that's been slowly degrading is also a waste of time imho.
If you haven't yet, try osmo polyx rapid. It's effortless to use and gives an excellent finish. I'd never go back to oil based varnish now.

Sent from my SM-G900F using Tapatalk
Edit- pretty much what ED65 said :lol: oops

 

[reck123]

thanks for the feedback guys. I will check out that osmo stuff next time round for sure, I got reccomended the polyurethane oil as i was doing some waterbased screenprinting on wood, and was told this wouldnt interfere with the waterbased ink. but the osmo stuff appears to be oil based to so should be fine.

Should i start sanding the boards at 320 after a few more days of curing and then progress to higher more microabrasive grits to get that smooth to touch finish?

 

[reck123]

just an update. i ended up using some vf300 mirlon Fine Scuff Roll. rubiing over the shiny spots. and it took them away and produced a smooth albeit matt finish. thanks for the help guys