This is a hybrid of a post I put on Facebook and a lesson plan I wrote out for my trainees, it’s still under review and will constantly as I learn more myself.
It is really centred around Woodturning but I’m quite sure it passable elsewhere. I wrote it out from what I have learned and picked up over the last 16 years of turning and it’s all my way way and my opinion I’m sure there are a lot of others out there. Sanding, I’ve found isn’t really taught in detail so I hope this will help someone out there.
SANDING….. How good is your finish?
Did you know when your sanding out that scratch or tool mark your not actually sanding it out?
You are actually sanding everything else away to the bottom of that scratch or mark or tear out..
Sandpaper is a tool for removing wood (or other material) and sanding is a skill, this tool and skill has to be learnt and mastered just like using a skew or gouge or any other tools.
What I try to teach my trainees is to get the very best “off the tool” finish as possible, try to get rid of all torn grain, scratches and tool marks before sanding starts. When you start sanding select a grit that matches your imperfections not your nice smooth areas, sand till the only imperfections left are the ones caused by “that” grit then move on to the next finest an so on and so on.
Each step you should try to make it so the next one is easier, from the roughing out process to applying your final coat.
Steps to a great pre-finish surface
Turning is better that sanding, turn your piece to the shape you want but leaving enough wood for “finish turning” this where you go through a process of fine and ultra-fine cuts and or scraping till a smooth non-faceted surface is achieved or very close to it.
Sheer cutting leaves the ultimate surface but can’t always be used, sheer scraping with a gouge is the next step and then with a skew as a scraper, a cabinet scraper can also be used at this stage and finally with an actual scraper. These tools can be used as one or altogether to achieve your goal surface.
Stiffeners like cyanoacrylate (CA or superglue) pva wood glue, sanding sealer, shellac or varnish can all be used to allow soft flaky fibres to be cut. Filling the voids of torn fibres often result with a dirty or miss-coloured spot that can bring down the whole piece.
Starting to Sand
Now you have the piece as near perfect as possible look for the defects that remain and choose a sandpaper grit that matches the imperfections closest then drop back one grit. (If you think it’s 320 surface use 280) then sand oscillating your hand, better still try to emulate the action of a random orbital sander. (When turning spindles stop the lathe at the end of each grit and sand with the grain) Do this over the entire area to be sanded till the only marks that remain are those made by the grit you are using. If you find imperfections that are not being removed STOP wasting your time and go back one more grit or even back to the cutting scraping process. Now continue this process to about 360 grit. This is about the stage where your finish is thought about. You may now coat with a sanding sealer if desired/required, wait till cured then sand off with 360 grit.
Final sanding and starting to finish
If you have a low penetration finish that sits on top of the surface such as polyurethane or wax and polishes you can continue sanding through the grits to at least 600 but all the way to 2000 and beyond if you wish but remember if you find a scratch or mark don’t be lazy go back as many grits as needed to remove it then proceed.
If you have a high penetrating finish like oil or you are staining the piece you can start applying it now while the surface fibres are open and uniform …. Further sanding will start to burnish the surface closing the fibres, normally unevenly preventing deep penetration in some areas and allowing it in others causing a blotchy effect. Depending on your finish you can continue to sand through the grits.
Some oils are hard burnishing oils where you can wet sand with the oil finish till a high polish is achieved…. Check manufacturers instructions and research forums and tutorials for best results.
• the shinier the surface the more defects stand out.
• endgrain is harder to sand
• knots are “all endgrain” surrounded by side grain.
• you can stop the lathe to concentrate on trouble spots.
•power sanding makes life a lot easier
•power sanding is sanding with power~~~ same rules apply!
•prolonged sanding can cause an uneven surface due to early wood being softer than late wood (light rings/dark rings) especially in softer woods.
•sand with a slow rpm
•high rpm means more heat build up which can cause;
the surface to burnish unevenly,
the paper clogs faster and stops working,
Can discolour your piece,
Transfer colour to your piece,
•wet sanding can be used on very hard dense woods.
•wetting with water can raise the grain to give a smoother finish and maybe pop out bruised or compressed grain.
•old sandpaper is not a finer grade!
•use smaller sections of sandpaper and replace often.
•heat build up can’t always be avoided…use a bit of steelwool as a pad, it will also help to form contours.
• the more short cuts you take the longer you will sand (Generally).
•buy good quality paper and sanding pads, cloth backed is normally good quality.
•make your own sanding pads and drill attachments or hand rotary sanders …. Cheap and can be customised to suit; the job, type of sanding implement and type of sandpaper used.
•micro mesh is wonderful and easy! But only for finer grits, you still have to do all the steps to 380-400.
•polishing helps to find imperfections!
•sanding isn’t sexy!
•every time you cut or use sandpaper think of how you can make the next step easier!
•if you can’t get out a mark or blemish, think about putting a feature line in or for larger sections try texturing….. People do that to good surfaces so it should help a bad one!
-- There is always an opportunity to learn. . . .Steve Mcgrady. . Sydney, Australia.