Thursday, January 26, 2023

How to Prepare MDO Plywood Sign blank - Doc Guthrie's method

Intended for sign tradespeople, this entry can also be useful for illustrating to clients what their sign is made of and how much time goes into preparing a quality product. Additionally, a traveling tradesperson can share this information with a client who might wish to prep their own sign blank, for the purpose of cost or time efficiency. The whole process including dry time takes about three days.

    MDO stands for medium density overlay - it is the type of plywood used for exterior signs and is more than sufficient for interior signs as well. "Medium density" means the plies are compressed tighter and adhered with stronger glues than your average cheap sheet of plywood. "Overlay" refers to the smooth sheet of paper that is glued to the face of the plywood. It can be purchased single sided (paper only on one side) or double sided. Common thickness options are 1/2" and 3/4". I use 1/2" for signs that are to be mounted on a wall and 3/4" for signs that stand alone, e.g. A-frames or sandwich boards. Specialty lumber yards carry MDO - Home Depot and Lowe's do NOT.

step 1) Cut and sand the edges of the plywood.

Straight lines can be cut by running a circular saw along a ruler clamped in place, see photo. Put a scrap of wood between the clamp and your sign face so the clamp doesn't dent the face.

step 2) Spackle the edge - Seals the edge and keeps moisture out

We use Crawfords brand vinyl spackling paste in California. In dryer climates sandable silicon caulk works better. Apply using a 1" spackling knife and smooth it with your fingertip. Once all the edges are coated let dry 24 hours.

step 3) Sand and dust the spackled edge. It should now be smooth and dry.

step 4) Prime BOTH sides of the board;

Dunn Edwards Evershield water based primer is the best primer to use. Oil based primer is an option as well, see notes above.

I put the primer in a paper cup and pour it on the board laid flat then distribute it with a 4" short nap cotton roller.

I always start with the back of the board first. Roll parallel stripes the length of the board, then roll perpendicular to that, then roll diagonally from both directions, using decreasing pressure each time you change direction. This will give you the smoothest primed finish. Using the edges to handle the board, flip it immediately on to "paint cones" and run the roller along the edges, making sure they are completely coated in primer as well. Then I repeat the same 4-direction process on the front of the board. Time to let dry another 24 hours....

But wait, we need to clean the roller pad so we can use it again. Save excess primer by using your butter knife to "cut the cob"

also use it to scrape primer from the cup back into the can.

Rinse the pad in cool water until the water runs clear, then spin it on this mechanism into a bucket to dry it out..

Now you have a clean roller you can use several more times

step 5) Enamel background; here are the tools I use. For a single-sided sign you only need to coat the one side with enamel.
I start by using blue painters' tape to remove the excess cotton fuzz from the shure line 3" cotton roller,

Then I use my butter knife to fill my dixie cup with background enamel..

Now that my paint and my roller are ready, I pour the enamel in a zig zag pattern across the front of the sign and work the roller across it, one direction at a time. I pause once between directions to also coat the edge, then do the final direction, removing excess buildup around the edge and leaving a very smooth enamel finish to dry 24 hours.

Following these steps will give you a professionally coated and sealed sign blank that will last many years outdoors.

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