2017 April 2    Woodworking   Wood Finishes



Introduction


In "How To Seal Particle Board", we looked at using polyurethane varnish.  The basic idea was to create a durable finish that doesn't let it soak up spills. Or, that sudden rain shower when you've got the table outside for an afternoon.

Polyurethane may trap bubbles when you apply it, though.

Is there a way to avoid this?  Let's see.

Wood finishing can have some hazards, and there are no guarantees you'll be able to get a good finish on any particular piece of furniture.  Please read the Disclaimer.


A Quick Note

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In This Article

Vapors and Safety

Brush Selection

Technique

Does It Matter For Particle Board?

One More Thing

Conclusion





Vapors and Safety


If you're going to do poly finishes, get a respirator.  Just get one now.  It's one of the most important things you can have in your workshop.  (Grab some gloves too.)

The vapors from a poly finish might not seem that obnoxious, but they'll start to get to you after a while.  That type of respirator that I linked to seems to filter out most or all of the fumes.  (The room should also have good ventilation.)

OK, now let's talk about brushes.



Brush Selection


Experienced woodworkers use finishing brushes.  The better brushes are made with natural bristle, ox hair, or a mixture of both types;  try this link or this one for some possibilities.

You might read that disposable foam brushes are the worst thing to use for polyurethane.  They are said to increase bubbling.  This was something I actually learned through ignorant bumbling, because what did I know?  (Said I, after going through several cans of it... "Wait, you're not supposed to use a foam brush with polyurethane??")

But if you're putting a poly finish on some cabinet-grade particle board, a foam brush seems to work fine. 




Technique


First, avoid dipping the brush into the main can of finish.  Once you get bubbles in there, you won't get them out.  It's very easy to ruin a whole can of finish this way.  Been there, done that.

Pour a working amount of finish into some other container, and work from that.  Pour it carefully so you don't introduce bubbles.

Now, the foam brush.  Some people want to squish the foam brush up against the sides or bottom of the can.  Either they want to make it soak up more poly, or they want to squeeze out the excess before brushing it on.  Problem is, this will fill the poly with bubbles.  I know this, because I did this many times before I realized it was ruining the polyurethane.

Just let the liquid finish soak up into the foam brush by itself.  Let the excess drain off by itself.  Or if you help it along, go easy and don't churn air into it.  You might be surprised how well this avoids bubbles. 

When brushing, go slowly and in one direction for best results.  But the thing is, I don't always do that. 




Does It Really Matter For Particle Board?


As the finish sinks into that rough surface, it's going to displace tiny bubbles of air.  This will happen whether you're careful or not.

Most of the bubbles will just pop out of the liquid surface by themselves.  The little voids in the finish will level out... mostly.  You can sand with 220 and then 400 between coats if you want to smooth down the crater edges and bumps.

But maybe you shouldn't try to smooth the finish.  If this is for a work table, those little crater edges and bumps are actually helpful.  They make the surface somewhat higher-friction, so your work doesn't slide right off the table.

It really does seem to make a difference.

With no sanding and no special technique, you should be able to get a finish that looks like this.  Good enough for a work surface.  That was four or five coats.




One More Thing


If you really want to reduce the bubbling, try using thinned poly.  Mix one part oil-based polyurethane varnish with one part mineral spirits.  Apply this mixture as the first and second coats.  The thinned poly will let go of the bubbles more effectively, so they don't harden into the finish.

Most of the bubbling actually happens during the first couple of coats, but not all of it.  Still, I would switch to full-strength polyurethane for the last two or three coats, at least.




Conclusion


Polyurethane wood finish seems to work well for particle board.  For what it is, the finish looks very good.  The techniques that wouldn't work on fine cabinetry seem to work OK on particle. 

This was just a quick look at the problem of bubbling in polyurethane finishes.  I hope you found this article informative.  If you use the links on here to buy any of your stuff-- tools, work tables, whatever-- it helps keep this website on-line so I can continue to bring you helpful articles.

Thanks for visiting this page!





         


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