White lines on RP doors
Bob Niemeyer, forum technical advisor
Most manufacturers do not sand their panels very well, especially the cross grain. It depends on the final color, but with water stain, we flood the door, spraying on the stain and using an air nozzle to push stain into the gap and then after wiping, blow dry the gap area completely or stain will leech back out as it dries. This will also work with oil stain and a chip brush. On paint grade doors, we try and spec an MDF or stable panel that won't vary in MC and then we can caulk all the gaps.
What you're dealing with is a moisture related problem. If you have RP doors that are properly built with bushings and the panels are at proper moisture content prior to final assembly, you will rarely have a problem.
Make sure the factory that makes the doors is climate controlled. Make sure you have some way of controlling the moisture in your shop (50% RH, for most of the country except the west). Get a quality moisture meter.
If the panels are not close to 8%, don't spray.
Even with a finish on 8% MC wood, that wood will expand and contract seasonally. That is why the doors are built with a floating panel. Therefore, if the doors are finished and stained in the summer when they are bigger across the grain, in the winter in a heated house the MC will be very low and the panels will shrink, leaving the dreaded unfinished line. There's nothing wrong with the door.
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