Rustins Plastic Coating is a British made two part lacquer, which apart from being used for general furniture, has also long been tied to guitar making in the UK.
I first came across mention of RPC when I was a young teenager reading my first book on guitar making, Melvin Hiscock’s ‘Make Your Own Electric Guitar’. It has also become famous for being the finish used on Brian May’s guitar. When I was 16 or so I attended a lecture by the luthier Paul Fischer, who outlined a method for using RPC for the backs and sides of classical guitars. Since then I have also encountered other guitar makers who have used the finish. Despite its off-putting name, luthiers and craftspeople have been naturally drawn to the finish as it has several favourable qualities, such as: offering a flat, truly mirror like finish; it’s hard wearing; and unlike most lacquers, it can be applied by brush, meaning a truly professional finish can be obtained with minimal equipment and without a spray booth.
However, the general consensus is that it’s quite a finicky finish to use. I’m certain this is due to the instructions that come with RPC, which are not really aimed at the user is aiming for a high gloss mirror finish, such as on a guitar. For example, the instructions imply that all the coats could be applied in one day, though doing so would in our case would lead to a disastrous finish. Another difficult thing is filling the grain; the instructions say to use Rustins grain filler, which however only comes in light colours, leaving luthiers with the problem of how to fill rosewood and other dark coloured woods.
Since attending Paul’s lecture as a teenager, I’ve steadily been refining my technique with this finish, and have developed a reliable method to get excellent results every time. Many of the skills required are transferable to and from other finishes; for example, perfect wood preparation, and filling the grain. And the overall ethos is the same as with other finishes; the goal is a thin, level, glossy, flawless finish.
I’ve decided to write about my method here. I’ll go through some of the potential pitfalls too, and how they can be overcome. It also should be noted that I polish and buff my guitars with the body, neck and bridge still separate, avoiding the need to polish into any corners. I would also only use RPC for the back and sides of a classical guitar, as I think French polish is more suitable for the soundboard, and I don’t like lacquered necks from a tactile point of view.
One thing I have learned is that finishing is entirely its own skill, a difficult skill at that which takes many years to develop. I find the term ‘finishing’ to be slightly misleading, because when you start ‘to finish’, it is truly only the beginning.
Wood preparation:
As with any finish - lacquer, French polish, oil etc - wood preparation has to be perfect.
Before final sanding, all holes/gaps must be filled - for example, at the end of the frets, or between the sides and the binding - with a mixture of CA glue and wood dust. Before doing so, on some woods it’s advisable to first wipe the area with thinned sanding sealer, to stop any glue leaving a deep stain in the wood. Any dents can be steamed out at this stage.
After all gaps are filled, I sand everywhere up to 180 grit, in preparation to fill the grain. In guitar making, I never use grits lower than 120. On spruce and cedar I start at 180 grit. I sand methodically and thoroughly with the grain, aiming for a flat surface and removing previous grits’ marks, using a sanding block or rectangular pencil eraser as backing.
I am careful regarding the brand of sandpaper I use. For a time I used Hermes silicon carbide paper from Axminster, but found it would deposit small particles into the wood, visible in the final finish upon close inspection. I like to wear a dust mask and sometimes goggles when performing a long sanding task like this.
The problem of filling the grain:
The consequences of filling the grain poorly or with an incompatible filler can be rather disheartening; potentially resulting in orange peel/fisheyes after applying several coats in the case of complete incompatibility; alternatively, you may complete the finishing process, only for the finish to sink into the grain several months later.
Rustins suggest using Rustins grain filler. As I mentioned, the problem here is that Rustins grain filler only comes in limited colours, natural, teak and mahogany. It is suggested in the instructions that you can dye the natural-coloured grain filler with Rustins oil based dyes. This suggested method doesn’t really work, it also thins the grain filler so it’s ineffective.
In 2020 I built a steel string guitar for a friend, and was delighted with the guitar overall, including the finish. However, upon seeing the guitar only a couple of months later, I was surprised to find the finish had sunk quite dramatically into the grain. Even though my friend didn’t see what the big deal was, I asked if I could take the guitar back to refinish it. It was then that I decided I would resolve the grain filling problem once and for all.
The solution of filling the grain:
After some experimentation, I found that it is indeed best to use Rustins grain filler, but you need to dye it yourself, and not using Rustins oil based dyes.
Instead, I use Dartford’s metal complex wood dyes. This dye comes in the form of a powder, and is available in all the colours you could wish for. I chose the metal complex dye over others in the Dartford range because it has a high degree of lightfastness, meaning it won’t change colour at all with age.
To dye the grain filler, first put the required amount of grain filler in a container to one side. In a separate container, mix a small amount of your chosen Dartford’s dye with enough alcohol to completely dissolve the powder. You have to wait about 30 minutes to be certain it’s completely dissolved. Then mix with the grain filler. If the dye isn’t completely dissolved, individual particles of dye can ‘bloom’ once finish is applied.
Before applying the grain filler to the wood, I apply an extremely thinned sanding sealer to the wood. This not only stopped the surface of the wood from getting dyed, but also highlights any scratches or botches of glue I missed during the wood preparation process.
I apply the grain filler with a cloth, and with a couple of minutes I wipe off the excess. I like to apply the grain filler at a fairly thick consistency - I don’t thin it much with white spirit for example - as I find the thicker consistency fills the grain better. It’s important to be thorough, bearing in mind that a flat surface is key to a good finish, and that any dips/unfilled pores will cause problems later on.
After a couple of hours of drying I sand away the excess grain filler and the layer of thinned sanding sealer. This is the final sand and it needs to be thorough and methodical. The grain filler clogs the sandpaper; I like to get rid of most of the excess grain filler with 120/180 grit. I then remove the 120 grit scratches with 180 grit before finally moving to 240. It’s important not to sand too much, or you’ll expose new unfilled pores. I sand to higher grits on end grain areas such as the heel or top of the headstock. When sanded to 240 grit it’s reading for finishing.
Before commencing finishing, the grain filler will need to dry at least 12 hours, but the longer the better. It’s important to remember that RPC isn’t a forgiving finish, and any flaws will be magnified. So it’s important to be thorough with the grain filling/final sanding.
Brushing the lacquer:
I’ll describe my process for brushing lacquer below. I brush the back and sides while the soundboard is still unpolished, and the neck and bridge are still unattached.
It’s important to try to control as many variables as you can when finishing:
I buy a new set of brushes each time, 1”, 2” and 3”. I wash them, let them completely dry and check them for any loose hairs. I used to get fancy brushes, made with faux or real squirrel/wild boar hair. However, I found they deposited hairs sometimes during the process, and have now found a far cheaper brush set which works just as well, without losing hairs.
It’s a good idea to mix only the required of RPC each day.
I keep the windows open. It’s good to have ventilation for your own health, and for the finish to offgas effectively. It’s also important to wear a mask.
The temperature should be above 17 degrees. When it’s colder it doesn’t dry as quickly. The humidity should be below 60% rh.
I used to use a wide 3” brush, but now I prefer 2”, at least on the sides, as I find it’s easier to brush-out the lacquer and get a flat finish.
On brushing technique, I would say:
Keep in mind that the goal is a flat, even, thin surface.
Don’t overload the brush with lacquer, or you can get ‘runs’.
Don’t work too slowly; keep the brush wet, or it will start to ‘drag’
For every coat, brush in a different direction; with the grain, diagonally across the grain, across the grain etc. This stops lacquer from building up more where brush strokes consistently overlap, and ensures even coverage. The end result is a flatter surface.
The flatter and more even surface you get, the easier the following stages of the finishing process will be.
A first precautionary coat may be applied with thinned down RPC (thinned with Rustins RPC thinners). This coat allows you to see if there are any flaws in the wood preparation, and if so, it’s possible to spot-repair them when this coat is dry enough. I sometimes skip this step, if I’m confident in my surface preparation. Once the finishing process is properly underway, it’s not possible to go back and repair a mistake in the wood, without starting the finishing process over.
Timing is very important with RPC. My usual schedule is to put on one coat in the morning and one coat in the evening. With the exception of the first day where I find three coats is possible. On the first day leave at least 3 hours between the coats.
If you don’t follow these timings, it can cause problems with the finish. If you apply too many coats too quickly, gas from the lower coats can’t escape, causing problems in the surface, visible in the final product. Also, after 18 hours, the lacquer enters a ‘critical period’, where it won’t accept another coat without the surface ‘wrinkling’, and has to be left to harden for a few days before continuing. So it's best to set aside a few days and just follow the schedule. Part of RPC’s reputation for being finicky comes from the unpredictability of this critical period, but I’ve found following this routine gives me consistent results.
Between each coat, I ‘denib’ with 1200 grit sandpaper. I don’t actually sand properly, I just knock the shine off the surface, removing any lint and revealing any flaws in the finish so far. It also makes things easier when applying the next coat as things are more visible and the brush doesn’t grab so much.
In total I aim for 6 coats, so in just three days, the finish is on the guitar.
Letting the finish harden:
With the finish applied, now the finish has to be left for at least 5 days to harden, in a room which is warm and has some airflow. I like to have my dehumidifier going and the window open. Once again, ignore the official instructions saying you can sand and buff after 24 hours.
It’s also perhaps advisable to let the finish harden longer. Firstly, the harder the finish is, the easier it will be to sand flat without problems. Also, the finish will continue to offgas and shrink for several weeks, so a better finish is obtained if you sand and buff it after the off-gassing/shrinking is complete. Another reason is that it can be a problem to deliver a still off-gassing product to a customer. For example, if they kept the still off-gassing guitar in its case, the finish wouldn’t be able to off-gas and would actually crack. So it’s safer to give the finish more time to harden rather than less. In the rush to get products/guitars out the door, it’s easy to fall into this trap. Far better to take the time and do it right.
Wet-sanding the hardened lacquer:
I soak some 1200 grit wet-and-dry sandpaper overnight in some water with a bit of washing up liquid. Then, with the sandpaper wrapped around the rectangular eraser, I set about levelling the finish. You know when the finish is flat when all the shiny spots have disappeared. My strategy is not to dwell too long in any one place. I try to sand until the finish is starting to look flat, but there’s still an even spread of shiny spots everywhere. Then I change to 1500 grit sandpaper and completely flatten the finish. Once an area is flat I don’t sand it anymore; if one is reckless here, it’s easy to sand through the finish. Any flaws in surface preparation also make sanding through more likely here.
Any dips in the wood resulting from inadequate surface prep would have to be left at this stage, as trying to level the finish until they go away would result in sanding through the finish.
The wet sanding process is a task I imagine could be vastly sped up with the use of a random orbital sander or similar tool. However, at the time of writing I’ve only done it manually. That back is quite simple to level, the only challenge being perhaps any build up of lacquer along the edges. The sides are more finicky.
Buffing the lacquer:
I buff by hand using rags cut from a 100% wool shirt and some polishing compound. I do the bulk of the work with T-Cut polishing compound. It takes a lot of elbow grease, working small sections at a time, but I remove all the scratches left by the 1500 grit sandpaper and am finally left with a mirror finish. You have to work with fairly vigorous circular motions to get the mirror gloss; it’s certainly good exercise. It is difficult to be thorough and remove all the scratches but it is worth it.
I use one piece of cloth for applying the compound, and another for removing it. It’s worth laying down a piece of kitchen paper, on which to put the cloths when not in use; it would be frustrating if some small hard particle got on one of the cloths and scratches the finish your trying to buff.
