If you sand wood past 240 grit sandpaper, the wood dust becomes so fine that it starts clogging the pores of the wood. This prevents stain and finish from soaking into the wood properly, risking a poor finish job. Alternatively, if you sand too much in one single spot, you might create a divot in the wood.
If wood gets over-sanded in one spot, it starts to become uneven, with a distorted sort of appearance. This can happen to even the most experienced woodworker. Usually this starts simply by attempting to sand out a discoloration, defect, scratch or gouge.
What happens when you sand wood too much? Sanding the wood in one single spot will begin to appear uneven and a little bit unbalanced with the other areas. This kind of error is possible for anyone regardless of their level of woodworking experience.
Over-sanding
Sanding too much in a single place can leave your floor looking uneven or distorted. Usually, over-sanding occurs either because you lost your focus or because you were attempting to sand down a knot, bump, defect or just a particularly hard patch of wood – and went a little overboard.
Even after sanding, many of these woods will feel fuzzy or slightly rough after the first coat of paint or varnish has been applied. This is because the coating wets the grain and causes the fibers to swell slightly, ruining what seemed to be a perfectly smooth finish before you started.
Unfortunately, floorboards are only so thick. Too much sanding can wear them down until they're too thin.
These back marks are called metallic discoloration, or iron stains, caused by elemental iron reacting with phenolic chemicals in the wood, forming black iron tannates. Iron stains require the presence of iron, water, and wood and will not occur if one of these elements is absent.
Sand closed grain woods with no finer than 180-grit. Sand open grain woods with 120-grit followed by 150-grit, then 180-grit.
Ensure that all sanding residue is removed by wiping your whole piece down with a damp microfiber cloth or sponge.
Hardwood flooring can be re-sanded up to 10 times, depending on how bad the floor is and how skilled the person doing the sanding is. It costs about $2.50 per square foot to refinish a floor, plus $50 an hour for patching and thresholds.
The Golden Rule of Sanding
The golden rule for choosing your sequence is to never skip more than 1 grit. For example, if you start with P80, and need to finish at P240, rather than using every grit from P80 – P220, you can do P80 – P120 – P180 – P240. This sequence cuts out P100, P150 and P220.
Fine grits close up the pores of the wood, so if you sand to too fine a grit, less finish will soak into the surface. Generally, if you're using an oil or clear finish, you can stop sanding at 180 grit. Continue on to 220 grit if you're using a stain or dye.
Black spots are formed when the wood is exposed to metal and moisture. Like a wet glass or leaky vase. This causes a chemical reaction from the tannic acid and water. The chemical reaction leaves black spots on your wood.
Wood that's exposed to the sun's UV rays over an extended period of time will darken and even turn gray. This weathering happens because the sun causes chemical changes to the tannins in the wood, kind of like oxidation.
DON'T: sand against the grain, UNLESS you feel skilled enough to sand diagonally or across the grain. DO: start with 20 grit paper and work your way up to 120 grit paper. DON'T: Go too high in grit or it can cause you to over sand your floors leaving them unable to absorb stain or poly adequately.
You really want to keep sanding until all marks and any shadows are completely removed. You need to sand your floorboards until they look brand new, even if you're staining the boards (unless they're going to be black or painted).
Depending on a floor's thickness and the experience of the flooring professional, a floor can be sanded around 4-6 times in its entire life. Hardwood floors can be refinished every 7-10 years, making the 4-6 refinishes more than enough.
For heavy sanding and stripping, you need coarse sandpaper measuring 40 to 60 grit; for smoothing surfaces and removing small imperfections, choose 80 to 120 grit sandpaper. For finishing surfaces smoothly, use extra fine sandpaper with 360 to 400grit.
Whenever water or any stain or finish that contains water comes in contact with wood, it causes the wood fibers to swell, which is called “grain raising” or “raised grain.” After the water has dried the wood feels rough to the touch, and thinly applied finishes also feel rough.
What is the most durable finish for wood? If you want to prevent damage to fine furniture, cabinets, or hardwood floors, oil-based polyurethane is the most durable wood finish available. This product is also water-resistant and UV-resistant.