Don’t Put Nitro Lacquer Over That Polyurethane Finish [Here’s Why]

Want to make your electric guitar really stand out? Well, one of the easiest ways to personalize your instrument is to give it a unique coat of paint.

And a popular way to paint and seal a guitar body is with a solid color polyurethane finish.

Polyurethane will wrap the surface of wood with a hard-wearing seal that helps to protect the timber underneath. And this stuff is tough enough to brush off scratches and abrasions to boot.

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But, what if you want to change things up and redo an already finished guitar… and give it a completely different color coat?

Adding yet another layer of polyurethane paint, (on top of the last one), will only bulk up an already thick polyurethane finish.

So, at this point some people think using lacquer is the solution. The solution being that, applying a quick coat of thin lacquer is an easy way to paint over your finished guitar.

That is because, while a lot of guitar manufacturers prefer to use polyurethane finishes nowadays, this wasn’t always the case. Instead, it used to be fashionable to paint guitars using lacquer. Or, to give this finish it’s full name, nitrocellulose lacquer.

Nitrocellulose lacquer, (also referred to as nitro lacquer), is a highly flammable solvent-based substance that’s mostly used to paint a range of musical instruments.

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Yet, it is not as durable as a polyurethane finish, due to the fact that lacquer is made of more brittle thinner stuff. And, even certain modern day guitars that are sold as nitro lacquer finished, tend to have a polyester primer undercoat on them all the same (note: that’s polyester, not polyurethane – which are two very different types of poly).

But, here’s the catch; there are chemicals found in polyurethane that make it hard for lacquer to stick to that finishing coat.

And why is that? Well, because many types of polyurethane finishes contain chemical thinners. These thinner additives help poly to dry and cure faster.

However, lacquer breaks down when it comes into contact with thinners. In fact, even dry and cured lacquer can be removed if rubbed with chemical thinners (often referred to as lacquer thinners).

And this is just one of a few reasons why you shouldn’t apply nitro over polyurethane.

So, keep reading to learn why that nitro lacquer needs to stay far away from your polyurethane finished guitar…

Don’t Put Nitro Lacquer Over That Polyurethane Finish [Here’s Why]

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OK, What Are Nitrocellulose Lacquers Generally Used For?

Nitro lacquer is a guitar paint. Its used to paint cabinets, desks, chairs, and more besides. But, this solvent-based lacquer is just one type of lacquer paint used on acoustic instruments.

Acrylic lacquer is a type of lacquer popularly used to repair automobile scratched paint jobs. But it can used on guitars too.

What Is Nitro Lacquer? Nitro lacquer is just short-hand for nitrocellulose lacquer. They are one and the same.

But, in the context of your guitar, it’s nitro lacquer thats the specific type used to give it color.

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Does My Guitar Have Nitrocellulose Lacquer? How Do You Know If It’s A Nitro Or Poly Finish?

You can test it yourself using a simple cotton swab and some acetone.

Acetone is a paint remover, and it makes quick work of removing even hard dry lacquer.

It, however, does not remove polyurethane or polyester (at least not if that poly has already dried and cured).

So, if you want to check, find a unnoticeable small spot on your guitar;

1. Dip a cotton swab tip in acetone.

Dip a cotton swab tip in acetone.

2. Press that bud tip onto the small spot on your guitar.

Press that bud tip onto the small spot on your guitar.

3. If that finish breaks down and dissolves, that means its nitro lacquer. And if it does not dissolve, that means it is a poly finish instead.

So, Why Can’t You Put Lacquer Over Polyurethane? [4 Key Reasons Why]

Problem 1: Reaction

Now, nitro lacquer dissolves easily when put in contact with paint thinners. Even dry lacquer still reacts with those chemical compounds.

And the chemical compounds in polyurethane will work just as affectively as any paint remover, when it comes to shrugging off a lacquer top coat.

Don’t Put Nitro Lacquer Over That Polyurethane Finish [Here’s Why]

Problem 2: Adhesion

Another problem lies in the smoothness of a polyurethane coat. There is simply nothing for lacquer to adhere to when you apply it.

Normally lacquer is applied onto wood grain, so it can soak a little into timber – giving it something to stick to.

But, not so with polyurethane. That plastic surface offers up no grit or bits for lacquer to blend into.

Even sanding that polyurethane finish – in order to produce grit for the lacquer to cling to – won’t be enough. Not unless you plan on sanding that polyurethane finish right back down to the bare wood underneath.

Related Post: Does Polyurethane Hide Sanding Marks? (7 Things You Need To Know)

Problem 3: Temperature Changes

If you manage to successfully get these two finishes to stick together, that doesn’t mean they always will.

These finishes expand and contract when the temperature gets turned up. Like, for example, when placed in direct sunlight.

And the way each finish moves, (due to heat), is very different. These differing movements will cause polyurethane to peel away from lacquer. And as poly peels, that lacquer top coat will crack in turn.

Problem 4: Scratch-Resistance

In short, polyurethane is more durable because it is a tough nut to crack.

Lacquer, on the other hand, snaps easily under pressure.

So, if you put that easily scratchable lacquer over poly, you risk having to deal with a lot of visible cracks – cracks that wouldn’t have otherwise appeared if you stuck to the original polyurethane underneath.

Related Post: Nitrocellulose Lacquer Vs Polyurethane: Choosing A Finish For Your Guitar

Don’t Put Nitro Lacquer Over That Polyurethane Finish [Here’s Why]

So To Sum Up, What Can You Put On A Polyurethane Finish Instead?

Honestly… nothing at all.

Polyurethane finishes (just like lacquer) are a top coat sealant. Which means, this stuff is almost always the last layer and final coat when it comes to wood finishing.

The only way to layer on top of polyurethane is to sand it off. And when it comes to lacquer, ideally you would sand the poly finish right down to bare wood, and then apply nitro onto that.

And even sanding – whether down to the natural wood, or simply for a little bit of grit – will pose a problem in and of itself. This is because all of those sanding scratches will show through the lacquer (especially if it is a clear lacquer product).

All of these problems and issues combined turns what at first appears to be a simple paint job, into an incredibly frustrating project.

Long story short? Save your guitar (and your sanity) and don’t put nitro over poly.