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Knot fill sanded smooth

More on preparation – sanding sealer and using an index coat

I’m building myself a “La Cabronita” style guitar in black, with a very nice one-piece alder body from Phil at GuitarBuild.co.uk.

I can’t stress enough the need for thorough preparation for a guitar finish to turn out well. Black lacquer is very reflective and shows every defect, so it is especially important here.

Sanding sealer

Sanding sealer soaks into end grain

Sanding sealer soaks into end grain

The first step is to sand the body well with fine sandpaper to remove any fuzzy edges from the machining process, then to apply plenty of sanding sealer.

Alder is a very absorbent wood and you’ll see it soaking into the wood, especially on the end grain.

I sprayed several heavy-ish coats until the end grain remained wet-looking, indicating that the wood had stopped soaking up the sealer.

Once I thought that I had plenty of sealer on, I let it dry overnight before block sanding.

Sanding showed up a few low spots and defects in the wood which I needed to treat.

The worst of them were a small knot between the control holes which could be seen as the sanding dust collected in it

and a depression in the edge of the body which remained shiny and unsanded.

Depression in edge of body

Depression in edge of body

I filled each of these with Ronseal 2-pack brown wood filler

Knot filled with 2-pack filler

Knot filled with 2-pack filler

Depression in edge of body filled

Depression in edge of body filled

and sanded smooth.

Knot fill sanded smooth

Knot fill sanded smooth

Edge repair sanded smooth

Edge repair sanded smooth

Afterwards, the whole body was given another coat of sanding sealer and allowed to harden overnight.

Index coat

I wanted to be sure that I had sanded out and/or filled every imperfection, so decided to use a technique I learned from my car-spraying days called an index coat.

An index coat is a very fine mist coat of black (or another colour) lacquer which is typically sprayed over a primed surface.

Index coat sprayed

Index coat sprayed

You can even use this on transparent finishes because you sand it off again.

The index coat highlights imperfections because as you block-sand your guitar body, it remains in any low areas such as the very slight depression around our knot

Index coat reveals unevenness

Index coat reveals unevenness

and a few tiny dings which would have gone unnoticed.

Index coat reveals small dings

Index coat reveals small dings

I continued sanding until no black lacquer remained so I was sure that the faces of the guitar were perfectly flat and free of the tiniest of defects.

I sprayed another coat of sanding sealer and sanded to 320 grit, ready for the colour coat.

20 Comments on “More on preparation – sanding sealer and using an index coat

  1. Fantastic advice thanks again Steve! Once I put the sanding sealer coat on my Jaguar body I started noticing a lot of dings and scratched that I just couldn’t see when it was bare wood, this approach sounds like it will do just the trick. I tried to solve it by sanding and just laying on more sanding sealer to no avail, so I hope my sealer coat doesn’t end up too thick, might sand it back a bit. Wish me luck!

    • Hi Mike, yes, don’t be afraid to sand it back so that it just fills the dips and is nice and thin everywhere else. If you sand through, just spray another coat.

  2. These are some pretty valuable tips! I just found your blog, as I am refinishing some guitars and basses for a hobby. Will read it quite often from now on. Thanks for your tips, Steve!

  3. Yes Steve got to agree with the other comments ,just to add I’ve sprayed 2 guitars before only as a hobby ,but without sanding sealer ,but what a difference sanding sealer makes ,on the other 2 I just put more primer on until it was flat, but sanding sealer (a) seals the guitar and (b) gets a really nice surface to add primer,colour, and clear to ,anyone thinking of cutting corners like I done in the past I recommend not to ,this paint is very good and flows nice and level to,thanks once again Steve for your great tutorials

  4. Steve, great info as usual.

    Quick question – does it has to be nitro paint for a index coat or any dark spray paint will do? I have a dark brown spray can from B&Q i used to spray a speaker baffle for my amp. Works?

    • It should be okay as you sand it all off of course. Also as the index coat is sprayed very light (almost dry) then issues of compatibility are minimised.

  5. When sanding the sanding sealer how far ‘back’ should one go? I’ve given the body 5 good coats. Thanks.

    • It needs to be as thin as possible whilst still covering all over and providing a flat surface. If you use an index coat as in the post above, you’ll see when it’s flat.

  6. I’m planning to do a pretty heavy relic on an alder strat body.
    Will the sanding sealer have the correct look when I do all the destressed /worn thru spots?
    Thank you for sharing your techniques.
    Joe NYC USA.

  7. It’s called a guide coat and the reason why we call it that is because it guide’s your sanding and yes it will show every little imperfections and that is why we use it, although you used black I would use the exact color that’s to be used on the guitar I do this because in the event I don’t remove every last little bit thats alright I’m not going to see what was left because it’s the exact same color, this helps me because in the sanding process we stay off all edges and corners these are the first places that if your not paying attention to what your doing get all the sanding sealer sanded off.

  8. How would I know what type of wood my body is? It’s quite dark in appearance and seems quite a tight grain? Trying to establish if grain fill is required?
    Thank you
    Alan

  9. GREAT FEATURE, ADDRESSES ISSUES I NEED RESOLVED, BEFORE DOING IT! THANKS! ROB.

  10. HI Steve
    just about to start a project with my son and we are aiming for a white burst effect… the body is really pale wood and we want to try to go from white at the edge with a gentle burst to the natural wood colour in the centre .
    Should we be using sanding sealer ? will it affect the natural colour of the wood.
    We are about to order the lacquers etc from you so any advice would be great.
    ps total novices at this!

    • Yes you should use sanding sealer as the base coat. It is to all intents and purposes clear and will not affect the colour of the wood apart from a slight darkening which all lacquers will do as it fills the air pockets in the grain. To get an idea of what this looks like you can wet the wood – that is the colour it will be under the lacquer/sealer.

  11. Hi have been given a part project, Korg ‘p’ bass. The body have been beautifully french polished, but needs a polyurethane sealer coat. What would you recommend? I have heard that water-based sealers would raise the grain and of course I can’t rub down between coats.
    In addition, I am converting the fretboard to fretless. I will use maple infill markers on the rosewood board. Could you recommend a sealer for the neck/fretboard.

    • Sorry but I have very little experience of using polyurethane varnish or french polish. The last time I French polished anything was woodwork class at school around 1971.

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