Polyurethane finished table top finish advice

[sihollies]

Hi all,
I am a woodworking hobbyist I was wondering if anyone could please advise?
I have recently started in a new place of employment, where there are 6 or 7 boardroom tables that were recoated several years ago by the handyman.
I am very sure it is a PU varnish, and it seems to have been applied in extremely thick coats .
It doesnt feel tacky to the touch, but if any paper or card is placed on the table top, it adheres to it, resulting in it ripping when it is removed, which obviously isn't ideal.
Knowing that I attempt a bit of woodworking, they asked if I could try and rectify the problem.
I was wondering if rubbing it down with wet & dry, or wire wool would help alleviate this problem?
The last thing I want to do is strip the tables and start again, as this would take forever.

I welcome your experience and input.
Thanks
Simon

 

[woodbloke66]

No real experience of this sort of thing, but it sounds to me that the thick coats of gloopy PU varnish may not have cured between each coat, bearing in mind that this sort of stuff takes hours (or overnight preferably to cure). Without the opinion of pro' polisher you may be on a sticky :lol: wicket; personally I would stay clear of this job until you've received more expert advice - Rob

 

[Sgian Dubh]

Much like woodbloke66, this sounds like a job that needs some experience and expertise. I don't think a bit of rubbing down with abrasives as you suggest is likely to fix the problem; your description indicates something hasn't cured properly in the earlier finish application, and very thick coats of polyurethane are a likely cause. Unlike woodbloke66, who reckons to have limited experience of this sort of thing, I do, and it wouldn't surprise me to find upon a close inspection that a complete chemical strip and refinish job might be the best solution.

So, unless you've got the either the know-how and suitable equipment, or perhaps just the bottle to tackle the job I suggest, like him, you give the challenge a polite but firm and wide body swerve, ha, ha. Slainte.

 

[sihollies]

Many thanks to you both.
I expected as much to be honest, but thought I would ask the question.
I'm definately not going to offer my services to strip it of the finish.

Many thanks again
Simon

 

[Phil Pascoe]

It seems odd with a finish that's years old. I'd offer to do it with wet and dry and microcrystalline wax - no promises. It works or it doesn't.

 

[Sgian Dubh]

It seems odd with a finish that's years old. I'd offer to do it with wet and dry and microcrystalline wax - no promises. It works or it doesn't.

I agree that you'd expect a very thickly applied oil varnish to eventually fully harden, but I have actually come across this strange phenomenon two or three times before. I can't explain it, and my guess is that overly thick coat applications somehow interfere with the curing process leading to the film drying somewhat 'gummy'. Oil varnishes cure through absorbing oxygen and the resin components cross-linking to form a tough film. The solvent in the varnish (white spirits) is there primarily to get the resin on to the wood, and this essentially simply evaporates out of the liquid varnish relatively quickly when it's applied to the wood. I can only conclude that a thick varnish film on the wood's surface somehow interferes with the oxygen/resin chemical reaction. Of course, all the previous is speculation on my part, and I suspect a wood finish chemist would be able to put us all right on the subject.

On the other hand, I'm pretty sure there are alternative factors that can affect a proper cure of oil-based varnish, and other finishes for that matter, e.g., high relative humidity, poor ventilation, chemicals in the air, contaminants under the applied film, e.g., silicone ... maybe, and so on.

Interestingly, if you've ever observed a spilled and rather thick puddle of varnish on a surface that's not been cleared up (as I have) I've noticed that although it's had plenty of time to dry, maybe even months, it also tends to be slightly soft and sort of rubbery in comparison to the same varnish applied properly to a wood surface. So, maybe my uninformed speculation isn't too far off the mark, but I really don't know, ha, ha. Slainte.