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I want to know how I can make the C side of ACX plywood have a more finished look by smoothing and filling. What should I use and how should I do it?

Background:

I am making french cleat for my shop and am using ACX for it. Since some of the cleats are going to be visible I want to fill the voids in the C side and then smooth it out to approximate the A side.

I may or may not finish the cleats but if I did it would be stain or paint. I am cutting the cleats by cutting the plywood in 5" strips and then setting the table saw for 45 degree and cutting the strips in half. When putting on wall one of the final pieces shows the A side and the other shows the C side.

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    Can you tell us what you're doing with this, and why you chose ACX? Are you painting or staining? For folks outside the US, this is ply with a fairly nice side (A), a fairly crummy side (C), and exterior rated glue (X). – Aloysius Defenestrate Nov 14 '18 at 2:14
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    Now that we see your issue from the edit, if you want to see the wood on the cleats save yourself all that work and just use the A side! You don't need to split strips and have to flip over one side so the bevel is in the correct orientation for mounting on the wall, showing the C side, Instead just cut them differently so you can use the A side exclusively. You waste a little more material cutting this way but it's worth it for the save in effort and the consistency in appearance. But if you paint you probably won't notice the voids after a couple of rolled-on coats with a sanding in between. – Graphus Nov 15 '18 at 8:31
  • BTW re. the variable quality of ply as I mention, even given the acceptable faults for C grade it sounds like like the stuff you have there is not great quality — you shouldn't have lots of voids to deal with (except on the edges of course but they don't matter). Over here with our equivalent of ACX it's rare to see a void on the secondary face, one full sheet might have only one or two minor ones but many have none. Plenty of knots and sometimes ugly fills (Bondo? epoxy?) but few if any voids. – Graphus Nov 15 '18 at 8:35
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Plywood is surprisingly variable for a product intended to be consistent and not all C-grade sides are the same by any means. If you can't select the plywood to minimise or avoid the worst defects possible on the C side you may need to choose how you finish based on the piece or pieces of plywood that you're actually dealing with.

Simply sanding up to a normal finishing grit, 180-220, will do quite a bit all by itself and followed by the application of finish (with possibly a small amount of additional light sanding during the process) may be enough to yield a surface you find very acceptable.

Note that this is just about surface quality. Knots, patches and seams are all possible (but very variable) and they can be visually disturbing, and little short of painting will do much to diminish them.

The choice of final finish here is completely up to you, wax (with or without a sealer coat*, an oil or oil finish, shellac, varnish, lacquer and waterbased finishes are all viable alternatives. And paint is also an option. Paint is a great equaliser, and can do the most to even up the appearance of the most knotty or patched plywood.


*This can be a commercial 'sanding sealer' or nearly any finish applied diluted, excepting waterbased finishes which are not normally used heavily thinned.

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