Re-finishing a guitar

June 5, 2014

Value for Money?       

I’d like to describe my method for re-finishing a rather battered old modified Strat, but before I do, I have a mini rant to get off my chest !

When prospective clients ask my cost for re-finishing say, a guitar body, I give them an estimate of about £180. Often,(not always), they seem to feel that this is WAY too expensive and that’s the last I hear of them !  I get the feeling that many of them think you just wave an aerosol spray can at the instrument and the job is done. This idea has been given weight by a number of sites offering re-spray aerosols and many guitarists are inspired to try a DIY re-finish job. Sadly the majority, unless they are incredibly patient and ready to learn, fall far short of the professional job they had hoped to achieve.

I have spent a long time learning the characteristics of different finishes, the techniques  to seal and finish certain woods, how to handle spray equipment, and how to rub out flaws in a finish and buff it to a high gloss. That didn’t happen overnight, as anyone attempting it for the first time will find.

The  Case in question.

A few weeks ago a Strat was brought to me to be given a 2K high gloss finish with a black base colour. Frankly it was in an appalling state. It had been given a black finish of some sort over clear lacquer, obviously a DIY job and then various paper motifs had been applied and lacquered over. The cutaways had been cut off and the body clumsily re-shaped to a sort of peardrop shape. On top of everything the body was covered in dings and scratches which had to be filled before the finishing could even start.

First step- remove all the applique and then take off all the finish down to the bare wood:

 

r01 Chemical strip

The best method for this was to use a chemical stripper, since the finish was reasonably soft. Modern finishes are far less straightforward. There are very few available chemicals that will touch a catalysed finish, leaving the options of using heat or scraping off with cabinet scrapers and sanding.

After the finish was off I came to a more resistant layer of lacquer which softened with stripper, but which needed more robust removal.

 

r02 Scrape down to wood

 

The tool I am using here is a cabinet scraper. It’s a rectangle of carbon steel which is ground at 45° on the longer edges. Using a hard steel burnisher, each edge is turned over to make it hooked. Used on wood, it creates very fine shavings leaving a surface which often doesn’t need any further attention. I’m using it here to take the finish off down to the bare wood.

 

 

r03 shaped scraper for curves

 

A rectangular scraper is ok for flat and convex surfaces, but difficult to use on concave surfaces. To get into the body cutout, I’m using one of my violin scrapers. It’s tear-drop shaped, so it forms a curve of changing radius allowing me to get into the tightest corners. By laying it over, I can scrape quite wide swathe – once you learn how to use it it’s a very versatile tool. I made all of my violin scrapers using the steel blade of an old saw, cutting and grinding each one to shape.

 

 

r04 Wirebrush control cavities

 

 

The control cavities had filled up with black grunge after the chemical stripping. This is always difficult to remove, but a mini wire brush mounted on a Dremel drive shaft made this fairly easy. It’s important to let the grunge harden, otherwise the brush just spreads a sticky mess round.

 

 

r05 Finish sanding

 

With the finish removed, I started finish sanding using successive grits from 80 down to 320. In the process I used clean sanding dust mixed with  CA glue (superglue) to fill the various dings and crevices. The process can be speeded up by using a CA accelerator spray. After spraying, the fill can be sanded without clogging the paper. I use an orbital sander on all the flat surfaces, but hand methods are needed to cope with the edges and any curved features of the guitar.

 

r06 Re-shaping edge profile

 

As the sanding progressed I became more and more unhappy about the body shape. I can only guess at the tools originally used to shape this, but it was clear that the sides were not perpendicular to the face and the amount of curve on the edges was inconsistent along the length of the sides.

A high gloss finish would only highlight these design flaws, so I decided to plane the sides into the perpendicular and then , used a router with a rounding over bit to cut consistent curves along the edges. This dramatically improved the overall appearance and with a certain amount of hand sanding I achieved the smooth transition from flat surface to curved edge so typical of the Strat.

 

r07 Shaped and sanded

With sanding completed down to 320 grit I started the finishing regime. The body is of close grained wood, probably alder or poplar, so no pore-filling was necessary. However  it’s always a good idea to seal the wood – this took a couple of wash coats of shellac, lightly rubbed down. The advantage of this is that shellac seals the wood and sticks to it tightly. Any finish applied sticks to the shellac, lessening the risk of finish chipping off.

 

After this I applied 4 coats of black catalysed polyurethane paint. Prior to this I had used an aerosol black paint which hitherto had always behaved well under clear finish. This time for some reason, perhaps a change in formulation, the black paint bubbled up as soon as clear lacquer was applied . and I had to strip the whole sorry mess off and start again ! Fortunately the finish was still soft and taking it back to the wood was a far easier business. By contrast, the PU paint behaved impeccably and stayed stable after the first application of clear lacquer.

Then began the  rather arduous process of building up sufficient depth of lacquer to allow it to be flatted down and polished. A common misconception is that spraying miraculously produces a wonderful gloss surface straight away. Not so – unless you have incredibly expensive equipment and an entirely dust free spraying area !

To build up the necessary depth of lacquer requires 12- 14 coats. so the routine is, spray a coat, leave it to dry, then rub out any runs or dust spots and apply the next coat. I usually do 4 coats a day and on the next day, I rub it out flat with 320 grit Lubrasil paper and start again. After the final coat I flatten with 400 grit paper and then spray a coat with retarder thinners. This creates a fine smooth coat which is still vulnerable to dust, but which serves to fill up the scratches from the previous rubbing down. This is left to harden and cure. With nitrocellulose, the longer the better. It’s polishable after a week, but if it can be left for at least 6 weeks it’s much easier to achieve that perfect finish. With a catalysed finish I find that about 5 days is sufficient, especially if it’s kept in a warm temperature.

The final coat is then rubbed down with 800 grit non loading paper. I use 3M gold Fre-cut paper. It’s not available in this grit in the UK so I have to get it from Stewart-McDonald in the States.

Then I switch to using foam backed pads with an orbital sander:

 

r08 100grit sanding

 

I start with 1000 grit then switch to 2000 and follow that with 4000. It’s best to use these with water and after use I wash out accumulated finish from the pads. The result is a deep sheen with hopefully all scratches eliminated. The high gloss finish is achieved by the next step.

 

 

r09 Buffing out

The final stage is buffing. The machine shown is a dedicated buffer. It looks a bit like an angle grinder, but angle grinders are far to fast, about 3000 rpm. The buffer has a variable speed control and I find speeds of between 1000 and 1500 rpm to be best.  Farecla G3 is used for the coarse buffing and burning is  avoided by spraying on a light mist of water to dissipate heat and provide lubrication. This brings the finish to a high sheen, but leaved swirl marks.

Ideally I would like to use a buffing arbour:

 

 

BUFFING-ARBOR

 

Sadly these are costly machines but can, if used carefully. reduce buffing and polishing time to about 1/4 of what I can presently manage.

The sheen is further deepened using a mop with Farecla G10 and for that final swirl-free deep mirror gloss I use 3M Machine Glaze. It’s a superb finishing compound with an added advantage that for complex areas, good results can be achieved by hand.

So there you have it – if anyone feels that I’m overcharging for the job, I’d like them to reflect on how much they would expect to be paid for the time spent – and of course I haven’t yet mentioned the cost of materials.

Support your local luthier, appreciate their hard won skills and pay up with a smile !

 

 

 

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