So you’ve just spent an hour or so finishing that wood project and applied polyurethane. So far, so good.
But after the polyurethane coat dries, the wood surface feels rough and not finished (just like it did before you applied the poly coat). What gives?
Well, if you’ve ever noticed your polyurethane coat feel rough or grainy? It’s not just your imagination, this is actually normal. So let me quickly explain why wood feels rough after a polyurethane coat.
If you’ve applied a water-based polyurethane, then the water found in the poly may have raised the grain of the wood. When wood fibers come into contact with water or moisture they swell, creating that ‘raised grain’ effect.
However, if you are using an oil-based polyurethane, then the rough texture may be the result of dust, dirt, or small air bubbles. As the polyurethane coat started to dry, light dust may have gotten trapped on the surface. Also, air bubbles can form if you’ve added an extra coat before giving the previous coat a chance to dry.
Now, if this has happened to you, then you’ve come to the right place! Because in this article, we are going to give you some simple tips on how to prevent raised wood grain problems from ever happening again.
Plus, we are going to show you how to quickly fix this common woodworking issue. So lets get to it…
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Why Is My Polyurethane Bumpy? | Oil Based Polyurethane Problem
There are two main reasons for a bumpy oil-based poly coat; air bubbles and dust.
This is a surprisingly common problem for beginners. Especially when enthusiasm can sometimes beat out patience as you wait for polyurethane to dry.
Generally, to get the most protection out of polyurethane, you will need to apply 3 or 4 thin coats of poly to wood. And patience is key during this process. Why? Well, before applying any extra coat, you always need to wait for the previous poly coat to dry first.
This can be a very slow process as each subsequent coat of polyurethane takes longer than the previous one to dry.
If you add another coat of polyurethane atop one that is only partially dry, (even just as a simple quick touch-up), then you risk trapping air bubbles underneath the surface.
Let the polyurethane coat dry so that any bubbles in this viscous liquid will release into air naturally within a few minutes.
Dust And Dirt
There’s no getting around it. Sitting around watching polyurethane dry is like… well, watching paint dry.
So, it can be tempting to try to speed up the process by getting some air flowing around the room.
But, while using something like a fan to blow air onto that still drying poly coat seems like a good idea, it’ll only cause problems. Problems in the form of kicking up dust and dirt from the environment all around it.
As polyurethane dries, anything that lands on the surface of the wood is going to get stuck in that hardening resin.
So, before during and after applying a finish, make sure you are working in a space that is dust free. It needs to be an aired out area, but not breezy.
Raised Wood Grain After A Polyurethane Coat | Water Based Polyurethane Problem
While oil based polyurethane is a thick solution, water based polyurethane is much thinner. Requiring fewer coats than oil based poly, this fast drying finish uses water (as opposed to solvents) as a base ingredient.
This is great when you want to speed up the process of drying poly. But it can be an issue when directly applied to natural wood.
The water found in water-based polyurethane is likely what’s causing the wood grain to rise.
You see, natural wood is like a sponge. Even the slightest hint of moisture will cause the wood fibers to expand.
Which is why before applying water based polyurethane to wood, you should always apply a seal coat first.
A seal coat – such as Zinsser Seal Coat – prevents moisture from getting underneath the surface. So it will do the job of preventing the water inside water based poly from soaking into the grain.
That way, the polyurethane can do the only job its supposed too… and that’s to protect your wood craft.
How Do You Fix A Rough Finish On Polyurethane?
Whether your problem is with dust, air bubbles or raised grain, the solution is all the same. You are going to need a bit of sandpaper and a lot of elbow grease.
If the polyurethane surface is rough, then simply sand it smooth using a fine grit sandpaper. A piece of 220 grit sandpaper (or even finer) will do in a pinch.
Run the sandpaper lightly over the bumpy finish until smooth. And wipe the surface clean of all sanded dust first before applying your next thin poly coat.
So, Let’s Wrap This Up…
Polyurethane is usually used in the finishing of wooden surfaces. It is a way of protecting the wood from water, humidity, dirt and even UV rays.
However after application, if you find that the surface feels rough to the touch, it shouldn’t be too much of a problem. In fact, it is a fairly easy fix.
Some fine grit sandpaper (220 grit or finer) should help you buff out those bumps and ridges… and get back on track with your project.