10-23-2010 05:49 PM
I suppose I will get many answers to this question, but how do you know when you have sanded enough? How do you know when to change to the next grit?
One trick I saw was to take a pencil and lightly scribble a bunch of lines on the board and then sand until they are gone.
What do you guys say? When is enough?
10-23-2010 06:33 PM
It's basically when you have achieved an even sheen that has erased scratches from the previous grits. If you stop sanding and change grits when that point has just been achieved, it is more efficient to work through all the grits, starting course enough to remove the surface defects on your wood. For many projects with wood that has been machined with sharp bladed jointers and planers, that grit may be 120 grit, or even 150 grit. If you are using a film finish you can probably stop at 180 grit. For wood that has been hand planed, I often start and end at 220grit if any sanding at all is needed. For an oil/varnish finish, there are benefits from sanding a bit finer, to at least 220, but on some woods to 320 or even 400 grit. Beyond 400 you may be harming the ability of stains and finishes to adhere evenly and tightly to the wood.
The key is to look carefully--with plenty of light and with light coming from a low raking angle. Wetting down the surface with naphtha can reveal sanding scratches that might be over looked. In particular, look for any cross grain scratches. These almost always appear worse the farther along you get with finishing, so avoiding them and removing any that have snuck in is important.
There is no law about working through all the grits. Since most people tend to sand a little extra to be sure, you may end up taking off more material than if you jump grits from your starting level to your finishing level. It will take longer, with more of the work done with finer grit. This may be particularly desirable to do when sanding a film finish that you need to avoid cutting through the top most coat.
10-24-2010 10:21 AM
Here is some info that may be helpful.
Sanding wood--hard or soft--beyond 220 does little more than burnish the wood making staining difficult. This is particularly true if you are using a pigment stain which sits on the surface and relies on "nooks and crannies" to impart color. Softer more porous woods can be sanded to to 220 but harder less absorbent woods may stain best if only sanded to 150. The best compromise is to aim for 180 grit.
A number of years ago a large cabinet/custom furniture shop I was involved with did series of adhesion tests with various finishes and sealers. As part of this test we explored adhesion based on sanding grit. We found about the same adhesion up to 180 - 220. Beyond 220 adhesion dropped off due to burnishing of the underlying wood particularly when non-linear machine sanders were used. This was tested on birch panels. We also found that the resulting smoothness of the first coat of finish was not materially affected by the smoothnes of the underlying wood for sandpaper grits between 150 - 220.. The smoothest surface substrate for final finishes was obtained by sanding lightly after the first coat of finish was applied and dry. Which makes the case for a thinned first coat of finish.
So our conclusion was that sanding beyond 180-220 was not necessary and could be actually detrimental.
But, most important was that there was a big appearance affect if the surface was not HAND sanded in the direction of the grain using the highest grit used on the sanding machine. A flat pad sander produced a much flatter surface than a ROS. However, both required final hand sanding with the grain for optimum appearence. If not hand sanded, swirl scratches could show. Final hand sanding using a sanding pad in the direction of the grain is a must.
To carry it one step further, sanding at 320-400 grit after the first coat and subsequent coats was the optimum. No improved appearence was noticed by between coat sanding beyond 400 for varnish. 400 was the sweet spot for thinner finishes. Between coat sanding was always done by hand whether for flattening or for adhesion.
I think you will find similar thoughts in the popular finishing books but YMMV.
Finally, the first coat of ANY finish will soak little shards of wood and cause them to raise whether the surface was sanded, planed or scraped. When the first coat of finish dries these hardened shards are what causes the surface to feel rough. Sanding with 320 paper will remove these hardened shards and subsequent coats will go on smoother. So, smoothness counts after the first coat of finish, but not much before that.
The machine finish determines the starting grit. Jointers, planers, belt sanders etc, should leave a finish that allows starting with 100 grit. From there, go to 120 grit and sand until the marks from the prior grit are gone, then move to 150 and finish at 180 grit.
Plywood is factory sanded to 180 grit. Therefore, it's best to not sand plywood except with 180 grit and sand by hand. Get the first coat of finish on and then sand with 320. That way you are sanding the finish, not the wood. This avoids sanding through today's very thin surface veneer.
10-25-2010 04:43 AM
I use the "scribbling" method.
It serves two purposes:
#1 Before I start sanding, I'll scribble on every surface (ends, sides, & faces) of every piece. If I have quite a few pieces, it's easy to miss a surface on one or two pieces. I do this every time I go the the next grit.
#2 I'll sand 'til the scribbling is completely gone. This way, I know when I'm down to "new wood".
Of course, that's just one man's opinion.
As far as when to stop going to a finer grit, the previous posts pretty much covered that.
10-25-2010 09:31 AM - edited 10-25-2010 09:32 AM
Howard Acheson wrote:
Here is some info that may be helpful.
- trimmed -
Great post Howie...
I've used your advice on this topic in the past and it definitely has helped me sort out the finishing schedule of my ongoing projects.