A replacement slat can look straight and sound, then turn furry under a hand plane. Before pressing harder or taking another pass, stop and look at the grain. The direction that feels natural at the bench is not always the direction the wood wants to be cut.
Check the edge of the slat as well as its face. Where the grain lines slope, imagine stroking a brush so the bristles lie down rather than stand up. Aim to plane with that laying-down direction. Edge grain is a useful clue, not a promise: knots and changing grain can make different parts of the same board behave differently.
Secure the slat on a stable bench with a suitable stop or clamps, supporting thin stock so it cannot bow away from the blade. Check reused wood for nails, screws and other embedded debris before cutting. Keep your hands behind the cutting edge and clear of the travel path, and start with a sharp, properly set hand plane taking a very light shaving.
Try a short pass on an area that still needs trimming. A clean shaving and a smoother surface are encouraging; lifted fibres, little pits or sudden resistance are reasons to stop. If the grain direction looks wrong, turn the slat end for end, secure it again and test from the other direction. Do not try to cure tear-out by bearing down.
If neither direction gives a clean result, check the blade sharpness and reduce the cut before removing more wood. For a small rough patch near a knot or reversing grain, careful sanding with a block may be the simpler choice. Keep checking the fit and thickness: chasing one blemish can leave a replacement slat thinner than the parts around it.
Practical takeaway: read the edge grain, secure the work and let one light test pass guide the next move. Once you find the cleaner direction, pencil an arrow on an end that will be trimmed off so you can return to the job without repeating the guesswork.
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