You’re adding coat after coat of Tru-oil to that guitar, still somehow it never seems enough.
Sure, you’re getting the smooth amber finish that you always dreamed of. But, Tru-oil still feels like it needs just a touch more protection, doesn’t it?
Don’t get me wrong, this is how it feels when you apply any oil finish. From Linseed oil to Teak oil, almost all oil finishes need some kind of protective top coat.
So it isn’t surprising to find oil finished wooden surfaces sealed with a coat of lacquer, epoxy, or even with the notoriously sticky shellac.
But, when it comes down to using polyurethane as a sealing top coat, can this durable stuff be easily coated over a Tru-oil finish?
Provided that you’ve given Tru-oil time to not only dry, but to also cure and harden, then you can apply polyurethane over it. However, you will need to lightly sand the Tru-oil film first to get the poly to adhere to it.
It all sounds more complicated than it actually is. So, keep reading to learn all of the key tips and tricks you need to know about applying that final poly coat to perfection.
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So, Can You Honestly Put Polyurethane Over Oiled Wood?
In short, yes you can put polyurethane onto any oiled finish… including Tru-oil finishes. What’s more, more often than not, its a fairly good idea to do so.
That’s because, while an oil finish might be durable, it is nowhere near as resilient as polyurethane.
Polyurethane is more scratch-resistant than oil finishes due to poly’s elasticity. After this stuff has cured, poly isn’t completely rigid.
While not soft, there does remain some measure of ‘give’ in the surface. That bit of give means that impact dents are less likely to leave behind a scratch.
Plus, polyurethane is more waterproof than oil finishes too. This is thanks to it’s ability to wrap wood in a plastic film that guards it from water spills.
But Should You Use Polyurethane On Tru-oil? Doesn’t Tru-oil Already Seal Wood?
Tru-oil makes wood much more water repellent, but it doesn’t seal it and make it water tight.
Think of oil finishes as more of a way to protect wood from the inside. They sink into wood and coat those wood fibers with moisture-resistant resin.
Now, Tru-oil builds up more of a film on the surface of wood than say a Linseed oil or Tung oil finish does. But, that resinous hard coat is easily vulnerable to water-damage (from say, water rings left behind by steaming cups).
So, if you are fairly sure that your guitar won’t get a more than a drop or two of water on it, you can skip over the top coat sealant step.
Otherwise, you should use something a little more robust to finish things off.
Applying Polyurethane Directly Onto Tru-oil
Polyurethane can be tricky stuff.
And without the right prep between coats, polyurethane will peel right off after drying and curing.
Which is why, (as a general finishing rule of thumb), whenever we apply multiple coats of polyurethane (poly over poly) we make sure we sand the base coat lightly first, before applying the next coat. This creates little bits of sanding grit that the top coat of polyurethane can adhere too.
And this rule of thumb also applies when we put polyurethane over any oil finish, such as Tru-oil.
Not all polyurethane sealers suffer from this potential peeling problem. But enough of them do that it is good practice to sand between coats all the same.
Top 3 Tips For Prepping Tru-oil For Polyurethane
Tip 1). Wait For Tru-oil To Cure
Do not try applying polyurethane directly onto a soft Tru-oil film. Wait for that finish to fully cure.
And, how do you know its cured? Well if it is dry, that means it is dry to the touch, but it is still a soft and/or sticky film. And you can probably still smell those VOC vapours coming off of it.
But if it is cured, that means it has now turned into a hard unyielding resin (and isn’t still off-gassing).
Tru-oil dries quickly, (in around 2-4 hours), but curing can take longer based on room temperature and other environmental factors like humidity.
Around 4-5 days should be enough time. However, if you aren’t sure it’s cured, then it doesn’t hurt to wait longer.
Tip 2). Use Very Fine Grit Sandpaper
The aim of sanding the oil finish is to rustle up a bit of grit for the poly to stick to. So don’t go overboard with this by scratching up the finish.
Simply stick to 220-grit to 320-grit sandpaper. And then lightly scuff the Tru oil surface in long steady strokes.
Tip 3). Apply Thin Coats Of Polyurethane
Tru-oil is a rather thin oil finish. So, it can be tempting to think that applying a thick poly coat is the best way to protect it.
But, the opposite is true. Stick to using a thin polyurethane coat to seal things off. You can do this by thinning your polyurethane by blending it with mineral spirits. A mix of 2-parts polyurethane and 1 part mineral spirit should do the trick.
Alternatively, you could cut to the chase and just use a polyurethane product that already comes ‘pre-thinned’ in the form of Wipe-on Poly.
Should You Sand Between Coats Of Tru-oil? No, you don’t need to. This stuff is thin enough, (and easy enough to rub right into lumber), that sanding between coats isn’t required.
Using A Wipe-on Polyurethane Coat Over Tru-oil
Wipe-on Polyurethane – to put it simply – is polyurethane that has already been thinned out for you.
Created from a blend of poly and mineral spirits, it is much thinner and coats on more smoothly than regular polyurethane. It’s thin enough that it can be wiped-on using just a lint-free cloth.
Which makes it perfect for applying over Tru-oil coats. You can simply rub on this stuff onto the oil finish without any problems.
Minwax has a fantastic Wipe-on Poly that comes in clear gloss, (so it won’t discolor that Tru-oil amber coat).
Plus, it’s no slouch either; it will dry in less than 3 hours, and cure in under a day.
So To Sum Things Up…
Polyurethane gets on perfectly well with any oil finish… and Tru-oil is no exception.
Just remember two things;
- The oil needs to be wholly cured first.
- The surface needs to be lightly sanded and prepped.
Stick to those two golden rules, and your guitar will turn out great!