How do I get a white background when I expand a canvas? I get a black and white grid that I can't us

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Which way it happens depends on whether or not your original layer is the opaque Background.

If you expand a canvas that contains a bottom layer with the capitalized name Background and with a lock icon on it in the Layers panel, the document background is not transparent, so the expanded area takes on the current background color swatch. If your background color swatch is set to white, then the expanded canvas area is white.

If you expand a canvas where the bottom layer does not have a lock icon on it, the document has transparency behind all layers, so the expanded canvas area is transparent, indicated by the checkerboard pattern. In this case, your choices to get back a white background are to flatten the document (lose all layers, leaving just the locked Background), or as others are saying, add a white layer and send it behind the image. (I like to use a Solid Color fill layer because it’s parametric/editable, not rendered pixels.)

You can do it as Trevor mentioned, or make sure you have a backgound layer - named background, then make sure white is your background color, then expand the canvas.

The grid you are seeing represents transparency.  You need add a new layer beneath the background layer, and fill it with white

With white added to new layer