Thursday, January 20, 2011

Lessons in Finishing


Having completed only a handful of instruments with sprayed finishes, it's not terribly difficult to identify flaws and aspects of the process I've yet to come to terms with. Having said that, it helps to keep things in perspective and remind myself occasionally that the perfect finish - if there is such a thing - is an impossible dream for us mere mortals, a fact apparently acknowledged by many U.S.-based builders who happily farm out the finishing of their instruments to a professional. I'm sure the health and safety aspects of working with solvent-based materials play a significant part in that decision too. Down here at the bottom of the world, however, I'm left with the task of identifying problems with my technique, improving my finishing processes, finding materials that suit my situation and hobbyist status and developing the most effective means to combine and apply them. Although it's been a laborious and sometimes frustrating process, I'm cautiously optimistic about my next guitar - but then I always am!

I've made some mental notes of late where finishing is concerned:
  • Do a thorough job when pore-filling. At this point in time I'm using epoxy, as do many others. Aside from its effectiveness in filling the pores, I like the way it enhances the figure and colour of the wood it's applied to. Briefly described, pore-filling consists of applying the epoxy to the wood with an old credit card or spatula, then removing as much as possible from the wood's surface, leaving epoxy only in the pores. It often takes several sessions, allowing the epoxy to cure and sanding off dust nibs and ridges before the next application, before all visible pores are filled, but it's worth the extra effort given all the hard work that's preceded it.
  • Wipe down cured epoxy thoroughly. A by-product produced as epoxy cures is something called "amine blush", a waxy residue that can have an adverse effect on subsequent coatings. Thankfully, a good rub down with a damp cloth, or one dampened with a mix of denatured alcohol and water is sufficient to remove it. From what I've read, it's worth spending a little extra on a good quality epoxy and applying it in a warm, low-humidity environment to reduce the impact of blushing. The manufacturer of the boat-building epoxy I use contends that their product is not prone to amine blush, but I tend to give it a precautionary wipe down anyway as added insurance.
  • Find a compatible sealer. Oily species such as rosewood can be particularly problematic and can adversely affect or even prevent curing of top coats. It's worth experimenting with sealers or barrier coats to find one compatible with epoxy and your chosen top coat material. While some builders sand the epoxy pore fill back to the wood, in which case an effective sealer becomes particularly vital when using any of the oily wood species, I choose to leave a thin layer of epoxy on the wood - excluding the soundboard. The epoxy serves to lock in any oils present in the substrate and the task remaining for me has been to find a effective product to act as a tie-coat between the epoxy and the top coats of KTM-SV. Adhesion of my top coats has been a recent concern, but hopefully I've identified a compatible set of finishing products as I described in an earlier post.
  • Find a suitable top-coat. Having set myself the long-term goal of selling my instruments, I've pondered the various features that a prospective purchaser might commonly be looking for. A durable, high-gloss finish is one them, and is one aspect of instrument building that continues to test me, particularly as I've vowed to avoid solvent-based materials such as the much-used but far more hazardous nitrocellulose lacquer.
  • Drop-fill early. Any small gaps in the bindings are evident once the first top coats are applied. It's worthwhile dealing with gaps and pin holes while there are plenty of remaining top coats to even out these repairs, the caution being that with only a couple of coats of finish in place, extra care needs to be taken so as not to sand through to the wood when levelling.
  • Level sand with the finest grade of abrasive paper which will get the job done, i.e., top-coats are necessarily thin and remnant scratches from coarser grades of paper are difficult to fill. With practice, sprayed finishes shouldn't need much in the way of leveling and, as I've become more comfortable with the settings on my spray gun, I've not found it necessary to level sand with anything coarser than 600-grit.
  • Final level sanding of the cured top coats can't be rushed, i.e., don't be in too much of a hurry to crank up the buffer! Patience and a generous helping of elbow grease are pre-requisites as the instrument is wet-sanded in readiness for final buffing.
  • Be careful when sanding the edges of the guitar, and especially so around the headstock, where it's easy to "roll-over" an edge with the sanding block and remove more finish than is necessary or, heaven forbid, sand through to the wood. I haven't sanded through yet, but I'm keenly aware of the potential to do so.
Cheers
Pete