Even though it has not been available for many years we all still use the term sandpaper when referring to any sort of abrasive paper that we will use for rubbing down wood or other materials. In fact this may well have something to do with the general term of 'sanding'.
These days 'sandpaper' and I will continue to use that term as we all know what I am talking about, is a paper or sometimes a cloth onto which abrasive particles has been stuck. There are many different types of sandpaper and the abrasive may be aluminum oxide, glass, silicone carbide or even industrial diamonds. Each of these grits have different properties which make one more suitable than another depending on whet we are asking it to do. I now routinely only use a couple of types of paper in my shop and around the boat. For most of my dry sanding operations I use aluminum oxide paper which is available in a wide variety of grit sizes as well as in sheets, rolls or cloth backing. I find the most economical way to buy this paper is in rolls which I can then cut off as I need them. Aluminum oxide is suitable for use either by hand or for sanding machines but for many of the random orbit machines you have to buy paper with a velcro back to attach it to the pad.
Silicon carbide is very hard, often black in color, and when stuck to water proof paper with a suitable adhesive can be used wet and this is my paper of choice when sanding varnish or paint ready for over coating. The water acts as a cutting lubricant and helps the paper from clogging or loading with particulates.
Abrasive is denoted by it's grit size, the bigger the number the smaller or finer the grit.
Thus a 40 grit will be very coarse and 400 grit very fine. These number hold true for every paper no matter who makes them as the grit or grain size is measured by it's ability to pass through a sieve of a certain size. The size will often nearly always be marked on the back of the paper.
One thing I would advise against and that is buying paper from the local DIY store. Often they will have a very limited range and as most folks shop on price they tend to only have the old fashioned glass paper. Buying this stuff is in my humble opinion a total waste of money. Cheap it may be but the grit falls off almost as you look at it and it does not last long at all before it fails to cut as it should. Either buy on line or go to somewhere like woodworkers warehouse or Jamestown Distributors who advertise on this site and get your paper there. It will be three time as expensive but the aluminum oxide paper, which is often red in color will last 10 times as long.
You will often see paper as being open or closed coat. This refers to how closely packed the grit is on the paper. An open coat paper is often better for mechanical sanding or on softwoods such as pine and a closed coat will work well on hardwoods or when hand sanding.
There are a number of additional resources on the internet and you can log on to mirka or 3M both of which offer help when choosing the right abrasive, just don't call it 'sandpaper'
Nice site Mark. I will be referencing it often. I've never heard about open and closed coat paper before. I can see where that will make a big difference when sanding different materials. Thanks for the info.
Posted by: Tony | August 12, 2010 at 10:42 AM
No problem and I am glad that you found the information useful.
Posted by: Mark Corke | August 12, 2010 at 06:46 PM
What number of grit should you use on mahogany before the 1st coat of varnish?
Posted by: Laurie Benson | February 12, 2011 at 11:29 AM
Laurie
As I have mentioned before and as anyone will tell you the best finish can only be achieved if the wood is smooth to start with. I usually start smoothing the wood with 120 grit then switch to 240 which will give you a silky smooth wood on which you can start your varnishing. The first coats of varnish will lift the grain to a certain extent so I like to sand lightly between coats with a 320 grit paper. As the coats build up on the surface, after about 3 or 4 coats sand the surface with 400 grit wet and dry paper used wet. Make sure that after sanding you wipe the surface with solvent and clean rags, then go over the surface just before you apply the varnish with a tack rag. Dust is the enemy of a good finish. Work inside if you can where it is warm and out of any wind.
Good luck
Posted by: Mark | February 13, 2011 at 02:05 PM