Hello Dave, I used to use exactly this – 3/4in birch ply – and the problem is it flexes too much. If it’s not on a perfectly flat surface it will fill whatever voids it’s sitting on and your plane sole won’t end up flat; I banana’d a cheap jack plane that then had to become my scrub.

I think a granite surface really is a good investment, when you consider how much money and time could be wasted in inferior options. A reliable reference surface can be used for checking everything, including straight edges and squares, as well as lapping plane soles consistently flat. Granite just doesn’t distort.

Mr Sellers put up a link somewhere to a cheap granite surface plate, shorter than a no.5 but good for smoothers, let me try and find it…