Cleaning dirty baltic birch ply (marine grade)

[Drew Eckhardt]

One side of my current project has picked up dirty lines perpendicular to the grain. They might be from a garage floor. They might have been in the wood. I don't know.

Before I start applying shellac and lacquer I'd like to fix it if I can.

Suggestions?

It doesn't seem to sand out - I hit it with 60-grit sand paper to level it out and paid a lot more attention to the bottom side where I wanted the solid birch feet to be flush and the stain appears uniform top to bottom.

The top is going to get leveled out with flush trim and round-over router bits.

[Thomas S Stockton]

Looks like the begining of sanding through the top veneer and what your seeing is a combination of glue and the next layer. Sanding plywood with 60 grit is kinda aggresive and you will quickly go through the top layer.
Tom

[David DeCristoforo]

"...sanding through the top veneer..."

That's what. In the future, never sand veneered ply with anything harder than 150 grit and even then you have to be careful. Toilet paper is thicker than the veneer on most commercial plywood.

[Michael Schwartz]

When working with plywood or veneered substrate be very careful because what you see is what your finished surface will be. Also treat it like a finished surface the whole time and protect it from dirt. This rule goes in affect from when you are picking out your sheet goods up until you finish the piece of furniture. I have messed up projects before because I chose to use a panel with a defect that I thought I could sand out, but ended up just sanding through the plywood trying.