Treating Rust with Phosphoric Acid

One of the bigger projects we have in the workshop is a Burns Vista-Sonic from the early 60s. Alongside a number of other issues, it has badly rusted metal parts: particularly bridge saddles and bridge plate. Because these parts are very hard to find we’re forced to make do with the old ones… and they’ll do the job well once we treat the rust.

There are a number of ways to tackle rust, but because these parts are so badly oxidised I’m going to use a chemical solution. This will ensure all the rust will be eliminated because it can get in every nook and under loose plating.

1. Acid Bath

Ideally, we’d disassemble the parts but, because they’re so seized up, I’ve had to dunk the saddles with the height adjustment grub screws still attached. We can remove them later once the acid has had a chance to work. I’m using  a 30% solution of phosphoric acid. You can get much stronger solutions of phosphoric acid, but it doesn’t seem to have any noticeable increase in its efficacy past a certain concentration.

I’m working outdoors to minimise odour and fumes, and have also taken the precaution of wearing protective gloves. I let the parts sit in the acid for an hour. You can leave it overnight, but there isn’t much going on after a certain point.

2. Strain

I strain the acid through an old t-shirt so I can use it again. Obviously, store this stuff responsibly. Mine’s locked away in the dark in a large glass jar.

As you can see, we’ve lost a bit of the zinc plating that was hanging off a saddle. C’est la vie.

3. Disassembly and Cleaning

Now I can take it to bits a little easier. I’m using cotton buds to apply acid to areas that look like they need additional work. In this case, all the tiny machine screws need a bit of attention.

Here you can see the very badly rusted bridge plate. The rust has been made inert by the the acid.

4. Neutralising the Acid

In order to work safer and quicker, I neutralise whatever acid remains on the parts by covering everything in baking soda. Make sure everything is coated. This will help dry the parts and bring the pH level of the acid down to a safer level. There may be some fizzling and bubbling! I then, carefully, brush the powder off using an old toothbrush.

5. Clean and Wax!

I clean all the bits up with a cloth or cotton bud and a little WD-40. I don’t want to soak any of the parts, really, so go easy on it. Try to work as dry as you can and clean off any excess with paper towel.

When everything’s clean and dry, you can decided how to proceed. If you’re painting the metal you can get on with that. Ideally, we want to apply some barrier to prevent rusting in future. Especially in this case as much of the plating has gone. Paint or lacquer is one way to go, but not really ideal here when we have moving parts subject to abrasion.

A compromise is to use a wax. This provides a reasonable barrier and adds a little lustre to our dull grey surface. I rubbed in a few coats and let it sit overnight then buffed it up.

I’m under no delusion that this will prevent rust returning to this unplated metal, but hopefully the process will not be quite so rapid.

6. Inspection and Reassembly

Here’s the finished product. Rust free and ready for reassembly. Come back in a few weeks, and hopefully we should have it back on the guitar!

The assembled bridge: