From the Drawing Board
Occasional ramblings from
a Small Craft Designer

by John Welsford

An abrasive relationship

A young friend of mine who found his calling in a boatbuilding apprenticeship at about age 25 told me a couple of years later that he “Never would have started boatbuilding if I knew how much sandpaper there was in boatbuilding” !!!! Most of us who do our own maintenance feel the same at times, it’s the only way to do the job but isn’t it awful stuff? Powered mostly by elbow grease and sweat , hard work even when you are using an electric sander, taking ages to do the job when you’ve a lot of surface to cover and leaving big scratches where they show through the varnish when you should have been much more gentle, the stuff is a real mixed blessing.
But have you ever been sweating away at a job while up on the hard, and seen a professional next to you breeze through a job that would have taken you days? The guy didn’t look like he was working hard and yet he was finished, cleaned up and out of there in time to catch the game on telly while you are left slaving away all tired and grumpy!

Its not just a matter of more elbow grease, at times it must be obvious that the “pro” is not working nearly as hard as you are and yet he is covering much more ground . What sort of sandpaper is the guy using? It seems an awful lot better than the stuff you got from the builders' supply place last season, or was it the season before? You keep it in the bottom of the locker with the tools, sure it gets rust stains on it from the other gear, and its a bit damp but its there when you want it , so why did he keep his sandpaper in a Tupperware box?

Sandpaper was once an accurate description of the product, crushed rock bonded to a tough paper backing by a flexible glue was only the first stage of development of a tool better known in industry as “flexible abrasives” and a very big business worldwide, a business that contributes to more of the everyday things that you use everyday than you might think.

However, when it comes to the tasks that recreational boaties use flexible abrasives for, there are some facts to be aware of and some points to remember.

While there are literally hundreds of different abrasive materials, using many bonding resins and a wealth of different backings, not to mention variations on coating density (open and closed coat are but two of many) and of course differing grit sizes, the list below covers most of the common flexible abrasives we use in boatbuilding and maintenance.

SAND paper. The “sand” forms the cutting part of the tool and can be made of any one of a multitude of substances, from real “sand” to exotics such as super saturated crystals. It is graded by running it through a sieve which gives us a grit size. The larger the number the smaller the particles. The number is the count of openings in the sieve per square inch and 36 grit is about the roughest stuff you are likely to see in everyday use. While at the other end of the scale 600 is often used for very light work in some wet and dry papers.

The nearest we get to the sandy original is

GARNET. Reddish brown or pinkish in hue, almost always on a paper backing Garnet is in fact crushed and sieved low grade (in a jewellery sense) gemstone rock. It is generally used for finishing work on furniture and interiors. I find it good for final sanding before varnishing as it tends not to scratch but remember that it is not suited to sanding anything hard (two pot paints, fibreglass etc). It does not self sharpen and on anything other than dry timber it clogs easily.

Garnet is a cheap paper, and one very easily degraded by moisture and in industrial uses is stamped with a time expiry date. If I am going to use it I buy fresh stuff and check that date as well as the sealing of the pack, I never buy sandpaper from an open pack!

ZIRCONA ALUMINA Usually bright blue in colour and on a paper backing. This abrasive has very good self sharpening qualities, as it becomes stressed by bluntening, it shears tiny flakes off the grit exposing new sharp edges, it should be bought in “open coat” if you have a choice. Usually used for power sanding applications, “open coat” is much less likely to clog in our applications.

ZA is great for robust work where high removal rates are required and gets good results on most surfaces but not good for fine finishing. Superior in modern boatbuilding where resins and composites are common. Very good for sanding the bottoms on fibreglass boats and not bad for sanding epoxy coatings.

ALUMINIUM OXIDE Brown, yellow or blue, this is a very good abrasive for general purpose work, apart from varnish it is ok for finishing work in appropriate grits, does a good job whether used by hand or on power tools. This is about the most common of the “sandpapers” available from the local suppliers and is the everyday workhorse of the type. There is some cheap “bargain basement” product around but my experience has been that it is well worth spending a little more to get a better quality.

SILICONE CARBIDE. Black or blueblack in colour, usually on a cloth backing this is a very robust abrasive normally used in very heavy industrial applications Actively self sharpening in the same way as ZA , almost always open coat as it is mostly used on huge sanding machines processing the likes of particle board and mdf, SC abrasive cloth is particularly well suited to heavy removal rates on fibreglass, non ferrous metals, rubber and synthetic compounds and is the most effective method I have come across for removing excess epoxy resin or other glues from a job.

I also have a whole range of shaped wooden sanding boards with 40 and 60 grit SC cloth bonded on, some of which have stood up to regular use for years.

EMERY CLOTH. For polishing and cleanup on ferrous metals, quite specialised and not useful in any other than the stated use. Has a uniquely rounded particle shape and is made in a very close (high density) coating which further specialises the product. Normally only used in engineering shops and never for wood or fibreglass.

WET AND DRY. Waterproof papers used for finishing work on paints, able to cope with very hard surfaces and with its very close coating and consistent grit size will produce a consistently flat ( no prominent scratches marring the finish) finish. Also good for preparing fibreglass for painting.

Keeping flexible abrasives between jobs is worth some thought, . REALLY careful people put some silica gel in the plastic box and refresh the gel in the oven from time to time. Damp sandpapers clog, do not self sharpen and the backing tends to fall apart, oddly enough this applies to WET &DRY as well as the dry papers.

It should be kept crisp so the tradesman with the airtight plastic container had the right idea, note that commercially, most flexible abrasives have use by dates and should be kept in a temperature and humidity controlled environment.

If you are a regular user of sandpapers you might find some of the most effective abrasive types hard to get “off the shelf”. It might be a thought to phone a major industrial supplier and ask if they can supply “ends of rolls” or “offcuts” of their big belts. This gives you access to grit types that you would not normally be able to get, (SILICONE CARBIDE being one), I did this some years back and took away half a car trunkful for a couple of sixpacks and not much money so now will not have to buy any for a very long time.

Choose your abrasive types well, use the appropriate type for the job and look after the stuff between jobs. Use closed coat for hand sanding fine work where clogging is not a problem, open coat everywhere else, particularly with power tools.

Select your grit size with care, use a tiny bit of dishwashing soap in the water in which you dunk your WET & DRY (helps if you use warm water too, not always practical but it can make a difference) and last but not least, some flexible abrasives have a coating of zinc stearate to promote self cleaning, there is some evidence that this chemical interferes with some paints so try to use products which do not have zinc stearate coatings.

SILICONE CARBIDE and ZIRCONIA ALUMINA abrasive particles are very durable, and are rarely thrown out because of bluntness, a fine bronze wire brush will clean the sanding residue out and leave a fresh abrasive surface. If you have gone to the trouble of shaping up some wood or making a sanding float then glueing the sandpaper to it (Ados F2 or a similar contact adhesive), you’ll want it to last as long as possible, and the bronze wire brush ( try an engineering supplies shop, not your local DIY place) is an important tool.

Sandpaper is a very important tool, it comes in a huge range of types, it needs to be used appropriately and kept in a proper environment. Used properly, with the correct abrasive, coating type and backing, kept crisp and dry, and with the right grit for the job it can be a very effective tool rather than a chore.