Hi – Some years ago I did a kitchen build using Birch ply for the cabinet and drawer faces. When I finished them with oil based poly, there were considerable light colored specks or ‘blotches’ in some of the grain, which I could only imagine was fine sawdust that had gotten lodged or trapped in the grain. It was bit perplexing, as I had sanded with an orbital with very good dust collection, followed by vacuuming the faces well, and then used a tack cloth as a final pass. The effect was not consistent across the entire face, and am wondering if it might be a result of the grade of birch ply. The ply, while from a professional lumber dealer, not a box store, is still of a cheaper grade. Or perhaps a result of a poor finish technique on my part, perhaps sanding too too fine a grit (I seem to recall 220 as the final). I’m about to do another kitchen in a similar material, and want to prevent this problem. Thanks in advance for any thoughts.

Summary: Birch plywood, sanded to 220, vacuumed and tack cloth, then Oil-Based polyurethane, no stain.

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I replied on Sawmill Creek. This forum is for professional woodworkers, not answers from professionals. I apologize if you run a business.

First off. 220gt is too fine. A tack rag on raw wood/finish is a bad idea. It's not sawdust in the grain. By looking at the images the material looks thirsty. ie......you didn't put enough finish on it. Also it looks like you didn't ease the edges very much if at all which is the first place moisture will get in. If I had to guess it's moisture getting under your finish into the wood.. Wood even slab doors move with changes in humidity, weather etc.....especially wood with not enough protection.
my2cts

Just a low grade birch plywood

Looks like cheap birch ply.

typical sanding schedule regardless of finish(paint, clear coat)
150 grit before primer sealer
240 grit before 1st topcoat
320 grit before 2nd topcoat

Some very specific products like oil based poly don't like 320 so you stick with 240 between coats. When staining woods you sand 180 after the stain to increase adhesion without removing the stain.

To me, after I enlarged the picture, it seems like the grain changed. Almost like it went into an end grain kind of look or even a knot. You don't see the longer grey/black rays in that area like you do in the parts that don't have that look. It doesn't have anything to do with moisture but with the TiO2 or Titanium Dioxide.

Looks like cheap plywood and the substrate is telegraphing through. Not uncommon on China made rotary cut ply.

Tissue paper thin veneer with glue seeping through the tissue paper thin veneer.