Washcoats and Sealing Before Finishing

Written by: James Krueger On: Mar 13th, 2019

So we have been hearing a lot of questions from you lately regarding guitar finishing and how to avoid the classic mistakes.  Well I have been in the wood and guitar industry for nearly fifty years, and I can tell you that I cannot answer this question in one 500 word blog.  Books have been written on the subject and there is more than ample info out there on the web, but for this month, let’s look at one aspect of finishing that can both solve issues or create it’s own set of issues….washcoats (or pre-stain or wood conditioner).

Wood is a porous material made up of winter and summer growth rings that are hard and soft respectively….trees grow faster in the summer and slower in the winter, so it stands to reason that the winter growth would be more dense and therefore harder.  When you add any type of figuring to the wood like birdseye, or flame or even spalting, you exacerbate the problems with finishing wood even more.  Whether you are staining, dying or just clear coating a piece of wood, the finish soaks into the softer grain faster than the hard and depending on the wood (for instance birch, basswood or many figured woods) can become quite blotchy.  Depending on the product and the material, we may want a very even color or finish, or we may want the figure to ‘pop’ out like a 3D design in the wood.  For hundreds of years, furniture makers have always wanted a smooth flawless finish and therefore came up with the idea of the ‘washcoat’.

Simply explained, a ‘washcoat’ is a product you use on the wood before staining, that evens out the porosity of the wood resulting in the stain/finish going on more smoothly and with a more consistent colour. Typically, it is made up of your finishing product thinned down to half or less of it’s full strength using the appropriate thinner.  For instance with lacquer, I used to thin my lacquer down to about 25% (so 1 cup lacquer, 3 cups thinner).  The theory behind the ‘washcoat’ is that  the thinned finishing product will soak into the softer grain and not into the harder grain before the thinner flashes off and the product dries. Once dry and lightly sanded,  this results in a more consistent surface which is ready to receive the stain or dye.  The ‘washcoat’ prevents the stain from soaking into the softer grains and creating an uneven colour.  Does it work?  Yup, it actually does, but it depends on the type of wood and partially how well that wood has been prepared.

They say that harder darker woods like walnut or mahogany do not require a washcoat and I tend to agree, but the benefits of washcoating are so great, that I just do it as a general rule.  The exceptions to the rule are figured woods like maple ash and spruce where you want to make sure the figuring ‘pops’ out at you.  In this case (like any other) a sample is always the best barometer….always keep a few scrap pieces of your project if you can so you can experiment with your finishes.  All woods are different regardless of being the same species or not.  A friend of mine who is a pre-eminent archtop builder, was preparing a highly figured maple top guitar for a blue dye (another story for the future). That builder had to take scraps and complete more than 50% of the finish with and without a washcoat to insure they got the right effect with the dye and the figuring.  Considering the guitar was sold for really a lot of money, the experiment with samples paid off!

What about just buying a wood conditioner or pre-stain from your local Home Depot?  Well you can, but I will caution you that some of these products are oil based and some are shellac based, but in either case, it takes a lot longer for the product to dry and for you to be able to stain that they tell you on the can!  I’ve said a thousand times that the secret to a good finish is always patience….rush the finish and you will probably be sorry.  Can you washcoat with water based products?  Well yes you can, but it’s a bit harder to determine the correct formula.  When you use water as your carrier (as compared to oils or lacquers) thinning down your water based finishing product can add to your difficulties since water does not dry as fast as thinner or shellac.  Of course, the stain will also not take nearly as well over a washcoat if it’s water-based either.  Check with the manufacturer of your water based product and see if they sell a washcoat or conditioner, or once again, start experimenting to find the correct formula for the product you are using.

Another question we have received in regards to our spalted tops is that the manufacturer of the bodies sometimes has to repair cracks in the spalted top with glue, and that stain or dye do not absorb into the glue.  Again a place where pre-sealing or using a washcoat can solve your problems….the washcoat soaks into the wood but not the glue, and when you apply the stain, there should be far less difference between the two.

This is no longer a 500 word blog, and I hope I have shed some light on this part of the finishing process.  Perhaps it will help you, and perhaps not.  In any event, I urge you to do your research….there’s tons of it out there.  The more you know about finishing the better, and the more the cardinal rule will sink in…..patience!  If you have any questions on this or any blog, just sign in and comment below…..