Washcoat Formula

[Robbie Collins]

Today I read Paul Snyder's article "How to Prevent Blotching using a Washcoat" at finishwiz.com.  I found his method for calculating the amount of thinner confusing so I came up with this formula.

The numbers come from his example. He starts with 8 oz of 2-lb cut shellac which he says has 16% solids (he's using Zinnser).  I don't have any loose shellac to measure the volume of, say, a half pound in order to check his math.

[Mark Fogleman]

Interesting. Never looked at washcoats that way but it makes good sense.  I use 600 grit to burnish end grain Mahogany to get it to match face grain color.  My experience with figured Cherry has always been challenging.  Some of the pretty stuff blotches with 1/4lb cut shellac.  It also blotches with application of alcohol or water based Transtint.  The best practice I've found is to spray 3-4 very light coats of 1/4lb blonde shellac instead of  brushing or padding it on.  Scuff with a grey scrub pad, spray the dye instead of brushing on, seal, tone with a glaze and then start the top coat build.  

[Robbie Collins]

Thanks for the insights, Mark!

As I understand it, end grain presents a cross-section of the phloem - the nutrient carrying tubules through which water (and solvents) wick. Sanding, burnishing, or washcoating all fill some of the tubules thereby lessening the effect of dye.

When you say "glaze" do you mean a colored, transparent finish coat or a gel stain?

If 1/2-lb cut is 5% solids then 1/4-lb is 2.5%. Have you tried using a thicker cut and fewer coats?

[Mark Fogleman]

A glaze to me is a pigment stain applyed after filling/dyeing/sealing a surface.  It is allowed to dry partially and wiped off.  It deepens and changes the foreground tone but does not hide the figure.  My favorite trick with Walnut is to fill, dye with a bright yellow or orange dye, seal w/shellac and glaze with a Van Dyke brown oil stain.  It creates a glow I can't get any other way with Walnut.

I will try to use the solids formula with a Cherry Media cabinet I'm working on.  I have used multiple coats of the 1/4 lb cut as a sealer for many years.  I doesn't add but a few minutes.

[Robbie Collins]

Thanks for the walnut tip...I'll be using some soon and will give it a shot. 

I'm a big fan of gel stains though I've never used them on a guitar. Here's a photo of the crown molding in my hall. I accidentally ordered poplar instead of clear pine so I bleached it with oxalic acid to remove the black, then chlorine bleach, and finally a two part sodium hydroxide/hydrogen peroxide. Then I used homemade walnut dye made from black walnut shells, then an oil based stain, spar varnish, a gel stain, then more varnish. You can't see it in the photo but it has definition and depth. A total accident, of course.