Q . Why can't I sand heart pine with the same procedures as oak or maple?
A . Antique reclaimed heart pine can sometimes be difficult to sand. Heart pine has sap or resin in the product, which makes sanding a more difficult job. Additionally, there are several things to look at when sanding heart pine. For example, how well is the floor installed? Is there much overwood? How well do the end joints match?
Most floor sanders try to sand the floor with the same grit sandpaper they use for a typical oak or maple re-sand job — 40 or 50 grit, sanding straight with the grain. The sap that is in the heart pine will load up the paper within 20 to 30 square feet of sanding. This makes for a long day.
Whether it's a new floor or a re-sand, as a rule of thumb, sand the floor on a diagonal of about 15 to 30 degrees with 36- or 40-grit paper. This will sand more aggressively than sanding straight with the grain. In extreme cases, you may have to use open-coat sandpaper such as 24-grit (or even coarser) to sand the floor flat. Once the floor is rough-sanded on a diagonal, then it is time to straight-sand. Sand the floor with a medium grit (40 or 50 grit) with the grain. Even though the paper will load up, continue to sand — while changing the paper frequently.
After the medium sanding is done, you will need to sand the edges. Using an edger with coarse-gritpaper (36 grit), sand all edges smooth and flat. Once you have completed the first cut on the edger, sweep, clean and vacuum the room thoroughly. Remember that keeping the floor free of debris, trash and grit will make a difference in how the final coat looks when the job is complete.
Before the final sanding with the big sander, edge with a 50- or 60-grit paper, feathering into the field about 12 to 14 inches. Since the pine is a softer wood, the edger may leave wheel marks in the floor. If not sanded properly, this will show up at the end of the job.
With the big sander, sand the entire floor with a 50- or 60-grit sandpaper, feathering into the areas that were edged. This will sand away anywheel marks that are left by the edger. On this cut, the paper will also load up, so sand as much as you can before changing paper. At this step in the sanding process, the floor will burnish from the paper being loaded.
After all sanding is done with the sander and edger, the hard work begins. Hand scraping all corners, wall lines, toe kicks, door jambs, and other areas is necessary to make the floor flat. Using a 60- or 80-grit sandpaper and working with the grain, hand sand all areas that have been scraped. This will smooth and blend edges from the field tothe walls. Clean, sweep and vacuum the floorsthoroughly. Now the floor is ready for screening. Remember, your final cut with the big sander was 50 or 60 grit, so you will need to start screening with an 80- or 100-grit screen. The screens will also load up and could cause swirls in the floor. Keeping the floor clean (as well as the screens) is important in order to achieve a good sanding job.
If the floor requires screening with an 80 grit, go back over it with a 100 grit to ensure smoothness. Now your heart pine floor is ready for finish. Remember, if you are using an oil-modified polyurethane finish, allow for extra cure time. Also, relative humidity and temperature will affect the curing of other finishes used on heart pine. Always check the manufacturer's recommended procedures before applying finish. Make sure that all finishes are fully cured before the next coat, as well.
• Do your first cut with the big machine on a 15- to 30-degree angle.
• The sap will make the sandpaper load up quickly — change paper frequently.
• Pine is a soft wood, so watch carefully for edger wheel marks left on the floor.
• Be prepared to leave extra cure time for finishes, especially oil-modified polyurethane.