“Natural Ash” Jaguar. Body preparation – Grain filling

The body for my Natural Ash Jaguar, is a 4lb 3oz, (1.90kg), two-piece assembly – cut from swamp ash. It’s apparently been faithfully copied from an original 1962 Fender body – even reproducing the original router plunges. Although my inspiration for a natural finish Jag actually featured an Alder body – many early Fenders were made from Ash, and the open-pored texture makes for a good, bright, but balanced tonewood. Since I’ll be trying out a completely new finishing technique – I wanted something which would both look and sound the part, but which wouldn’t break the bank. (Things can always go wrong). I’d been scoping out potential candidates for a couple of weeks, when I got notice of a “Flash Sale” at guitarbuild.co.uk. This particular example had been on my “possibles” list, and the promise of 25% off the usual list price, sealed the deal.

Latest delivery from guitarbuild.co.uk

Swamp (or “soft”) ash comes from trees where the roots remain covered by water. As a result – the wood is lightweight and more porous than “Mountain Ash”. There is often a characteristic figuring present – which on some examples can be quite bold. On this example the figuring is tighter packed and, although it runs roughly straight along the length of the body, it’s attractive, and the slight swirls and undulations will help visually define the three-dimensional shape of the body – especially over the arm cutaway.

The open-pored nature of the swamp ash makes it ideal for use as a tonewood – however, it does mean that the surface of the wood is covered with microscopic openings and grooves. These are the ends of, and sections through, tiny tubules which once carried the sap up the growing tree. Unless these are sealed and properly levelled – any attempts to finish the body, with any kind of smooth finish, will be doomed to failure. Of course – it’s totally impractical to fill every tubule – besides – it’s the open nature of these tubes which give the wood it’s characteristic, musical qualities. What’s needed, is a way to seal just the outside few millimetres of the wood. That’s where grain filling comes in.

Most wood finishes – be they paint, lacquer, oil, wax – have certain “filling” properties, but what tends to happen is that the finish materials tend to get drawn into the structure of the wood, a certain distance. Due to capiliary action – the thinner finishes will get drawn in the furthest – however, as they cure, this causes shrinkage, and leads to a certain amount of “pitting” on the surface. The resulting “pin-holes” completely spoil the effect of a smooth, consistent finish. They can sometimes be overcome by gradually filling-in, with repeated applications – but it’s a long battle, and the resultant finish coats can build up to be overly thick. Sometimes – and especially with paint finishes – a surface can be quickly filled and primed to allow for the final paint coats to be applied quite thinly, over the top. Paint primers and undercoats are usually slightly denser than paint, and are designed to “fill and build”, so that they can be rubbed and prepped back to a consistent smooth, flat surface. However – they tend to be opaque, and therefore this isn’t a method which can be used on a “natural” finish.

The effect of grain-filling on open-pored swamp ash

With finishes on most musical instruments, the best overall finish coats are always quite thin. Obviously, with “natural” finishes – where only transparent lacquers or oils can be applied – a method needs to be found which fills the pores and end-grain without resorting to an opaque “undercoat”. An approach which allows for the thinnest of fine finish coats to be applied on top of a stable base. “Sealing” the wood does this – and allows a fine, sandable filler to be applied, suspended in a clear shellac-based lacquer, which is then drawn into the outer “skin” of the wood – just where it’s needed. Once dried and cured, the fine filler essentially becomes a part of the outer surface, and any excess can be sanded away – leaving the fine pores filled, and the surface perfectly smooth. This provides the necessary fine, flat base on which further layers of finish can be applied, with much less tendency for the finish to sink. Grain filling is, essentially, the same technique as “sealing” – except the carrier oil tends to be thicker, and the proportion of filling material is much higher than with a sanding sealer. Woods with a heavy grain usually require grain filling – wheras finer grained woods, like Alder, usually only require sealing.

Because the swamp ash grain here is very porous and open – in preparing this body, I’ll clearly need to fill the grain first – but I’m also going to seal over the top of the filled surface. In doing so – I hope to provide the very best possible, smooth surface on which to apply a thin shellac, “French” finish. As with all finishes, (and especially here, since I haven’t tried any French Polishing before) – preparation is everything

“Natural” Ash Jaguar body – First stage of preparation – Raising the knap

The ash body is already finished to quite a high level. I reckon at least 320 grit, and that would normally be perfectly acceptable for grain filling. However – I always think it’s a good idea to ensure that the starting point is as sound as possible. Quite often, surface imperfections – especially at the endgrain – can be burnished down by the process of sanding, only to raise again once the work begins. “Raising the knap” – by wetting the wood – allows remaining surface imperfections to be exaggerated, by the fact that wetted wood swells slightly. The “knap” can then be re-cut and removed with fresh sandpaper. Distilled water can be used to raise the grain – but since actually getting the wood damp, internally, can prove counter-productive in the long run – I usually prefer to use naptha, which evaporates off without leaving a residue.

To remove the raised grain, I use a 360 grade, red Mirka “Mirlon” sanding pad, over a cork backing block. The solvent evaporates quickly – so each area has to be wetted, sanded and then left to dry out again. I work across the grain at first, and then finally finish along the grain. Now – although applying the grain filler will “wet” the wood further – there’s less of a chance that the grain will swell again and “roughen” the surface as the filler is applied. It’ll tend to go on smoother, and it means that there should be less of a build up of surplus filler, on the surface of the body.

“Natural” Ash Jaguar body – Second stage of preparation – Grain filling

The next stage of preparation is the actual filling of the wood grain. I’m using a spirit based filler by Rustins, which is a sort of liquid putty. A good deal of “natural” coloured, sandable filler is suspended in a thick drying oil, (perhaps linseed oil, or similar). The filler is let down with white spirit until it’s the consistency of double cream, and then is worked into the grain with a spatula or squeegee. I like to use an old credit card. It can be cut to shape, and is flexible enough to allow the filler to be forced into the open grain, without too much effort.

A small amount of filler at a time is applied, and worked into the surface – working against the main direction of the grain. As the filler carrier is pushed in, and as the solvent evaporates – the filler gets thicker, and so I divide the work into small areas – making sure I don’t miss anywhere. Once the filler begins to solidify, I begin to remove any obvious excess, and then move on to the next area. I’ll be repeating the application at least once more, and some of the filler will inevitably get sanded out again – so I like to concentrate on making the best of the filler while it’s still liquid, at this stage. That way – more of the filler will be be carried further and deeper into the pores of the wood.

For the radiused edges, and the open end-grain at the ends of the body, I prefer to use a dense cloth to push the filler in. Those fine, lens-cleaning type cloths work well. The end-grain usually takes a lot more filler than you’d think possible, and it helps to be able to wrap a cloth around the edges of the body – rather than scrape away at the curves and contours with the flat edge of a squeegee. The densely packed fibres of the cloth help push the filler into the pores. Once again – as much of the dried excess as possible is removed – before moving onto the next area.

Removing excess grain filler

When grain filling a guitar body – I tend to work on each side separately, and then work around the entire edge. Although care is taken to avoid too much excess filler drying on the surface – it’s the nature of the grain filler to dry leaving an opaque, chalky coating. This, if left to dry fully – will only need sanding off again. There comes a time during drying – perhaps up to an hour after application – where the carrier oil can be quite easily scraped away. This can be done with a dull scraper blade, (or another old, but clean, credit card). The excess filler comes away quite easily – but it’s important not to scrape away too hard. Also – to avoid scraping any semi-dried filler out of the grain – it’s best to scrape away the excess across the grain.

It’s also quite usual to get a few runs of dried filler, down in the pre-drilled holes and routs of the body. Although it’s not absolutely vital, (except perhaps in the case of the thimble holes on Jaguars), any excess can be removed quite easily when it’s semi-dry. Once all of the surfaces have been filled and cleaned back – the body is left overnight for the remaining filler to dry and cure.

“Natural” Ash Jaguar body – Third stage of preparation – Sanding back the grain-filled body

After the body has been left to dry, for about 18 hours in this case – the filler has hardened in the grain, and all that’s visible on the surface is a sort of general haze, with a fine tracery of newly-filled grain especially evident along the darker lines of figuration. Sanding the body now – this time along the grain – removes all evidence of hazing on the surface, and begins to finely level the surface. It’s important not to sand too hard – otherwise it’s easy to sand through the filled surface – so the emphasis is on removing all unwanted surface texture – fine scratches, witness marks etc. I use a 400 grit paper. Although I use a cork backing block on the faces – on the delicate edges, I wrap the paper in my fingers, and go gently, to avoid sanding through. It’s important to constantly check that the paper is unclogged, and as clean as possible.

The hazing of the surface acts a little like a paint mist coat and, quite often, can reveal a few more significant surface defects which may have been missed – such as slight indentations, or flat spots on the contours. Now is a good time to concentrate on building the overall quality of the finish, and so all flaws – however slight – are ideally lessened, or removed entirely, at this stage. In some circumstances this will mean sanding through the filler again, and back to bare wood – but there will be at least one more pass with the grain filler, and dealing with problems early on, is the best way to save time and effort in the long run.

Second application of grain filler

Once the body has been sanded back over it’s entire surface – I brush the body clean and take any remaining loose dust away, with a tack-cloth. A second application of grain filler is then worked into the grain, in exactly the same way as before – along, and against the grain. Forcing the filler into any remaining open pores. Special attention is given to any areas which may have required heavy sanding due to rubbing-out faults and flaws. Once again – the excess is cleared away by scraping across the grain, and then the body is left overnight for the filler to harden fully.

“Natural” Ash Jaguar body – Fourth stage of preparation – Burnishing the grain-filled body

After the body has dried once again – there’s another slight haze of surplus filler evident on the surface, with a few thicker areas, here and there, where the filler hasn’t quite been evenly scraped away. This time – the surface is lightly sanded back with some 800 grit paper, over a cork backing block – with special care taken, not to rub-through the finish over the edges. The surface is worked – again, along the grain – with the emphasis on removing just the surface haze, and on providing the smoothest possible, clean wood finish.

Once the surface has been sanded back – the body is smooth and silky – although it does feel slightly “greasy” to the touch. To finish off – I like to clean the body over, by rubbing with a piece of jute hessian. This is an old trick from my gilding days. The hessian is slightly abrasive, but without too much of an obvious “key”. By rubbing with the hessian – it’s possible to even-up the surface, and to begin to burnish the finish. However, the hessian also generates a little heat due to friction. (Don’t rub too hard – or the piece will literally “overheat”, and you may damage the finish). The jute in the hessian is also quite fibrous, and as the cloth is used, many small fibres tend to drop, (especially if the hessian is old) – so once the surface has been burnished – it’s a good idea to give everything a really good clean over again. However – before such a deep clean – it’s a good time to rake out all of the control and pickup cavities, and have a really good clean-up – if it hasn’t already been taken care of.

“Natural” Ash Jaguar body – After grain-filling

After grain-filling and burnishing – the body looks much smoother, and already has a nice sheen developing. Since I’ll be experimenting with a “French” Polish – I want the surface to be as flat and even as possible, and so to fully finish the process of sealing the wood – I now need to move onto the next stage of finishing…

Next time – applications of sanding sealer, followed by further polishing with even finer grit paper…

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