Sanding and Polishing Polymer
Finishing processes
Naturally, polymer has a fairly matte surface. However, part of its appeal is the way it can take on many different appearances. A proper finishing job can produce :
- an amazing deep mirror shine which brings out the colours and makes a piece look amazing.
- a silky, tactile matt finish which looks expensive and feels almost frictionless as you stroke it.
- a satin effect - a subtle sheen, like you get on a conker.
Unfortunately all of these effects rely on one thing: proper finishing. And that starts with sanding.
There aren’t many people who actively enjoy sanding. It’s laborious and time consuming, and it’s yet another assault on your poor hands.
BUT… if you’ve created something you love and will enjoy wearing or using, it will multiply your pleasure a thousand fold if you finish it properly.
There are two stages to finishing polymer: sanding up gradually through multiple grits of sandpaper to smooth and refine the surface. And buffing / polishing the surface to produce a high glass-like shine.
Sanding
How to approach sanding depends a bit on where you’re starting from. If your piece is badly misshapen or extremely “textured” it may not be worth trying, especially if you have thin sheets of a patterned veneer over a base clay. This is because you’ll very likely sand through the veneer and into the base clay, before you have managed to shape the piece to your satisfaction.
So the rule is to try to make your piece as close to the way you want it BEFORE it goes in the oven. It’s much easier to get clay to behave while it's still in its uncured state. If you cure it with lumps all over, and bits of scrap clay and dog hair stuck to it, you’re making life hard for yourself.
There are some good ways to cut down on the amount of sanding you’ll have to do before your piece is cured. These include:
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Deciding in advance that your piece is going to be textured in some way (eg by using texturing tools, or pressing sandpaper sheets into the raw clay to give it a pleasingly 'even' roughness)
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Using cornflour and a “pounce”to smooth the surface
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Putting layout paper over your clay and rubbing it smooth with your fingertips (also known as 'burnishing')
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Spending a lot of time patting and stroking your clay
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Wearing latex or nitrile gloves to avoid fingerprints and nail gouges.
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Using fibrefill in the oven tray.
However, in most cases you will still need to sand to get the full professional effect.
A cornflour 'pounce' can help reduce the need for sanding off fingerprints.
What sort of sandpaper do I need?
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I use the wet and dry sandpaper that Halfords (in the UK) sells for car body repairs. Last time I looked, they sold a handy mixed pack of different grits for about £6, which would do very well as a starting point.
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Usually the lower (coarser) grits like 240 and 400 take the longest and are the hardest work. They will remove relatively large amounts of clay and you can - carefully - remove lumps and humps and sharp edges with this grade of sandpaper, as long as you are careful not to sand too deep into your piece.
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As you work on these lower grits, remind yourself that you’re doing most of the legwork here, so that when you get up to higher grits you’ll only need a few seconds or a minute or two on each piece.
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At the higher grits (800+), you’re simply refining texture rather than fundamentally changing the shape of the clay.
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What you're aiming for: A piece that feels consistent to the touch – and it will be touch rather than sight that you use in this process. (It sounds weird, but if you put the piece to your lips, you can tell whether it needs more).
How to sand...
- Big bowl of warm water.
- Tiny spot of washing up liquid.
- Wet and dry sandpaper pieces, starting at least 240 and going up to 1000 or 1500.
- Put your pieces in the water to warm up.
- Put a towel over your knees and another one under the bowl to catch drips.
- Plonk yourself down in front of the TV (it takes long enough for you to get very bored if your brain isn’t occupied) and let your hands do the work.
How do I know when to move up to the next grit?
Short answer: Experience!
Long answer: At first the piece will feel slightly grippy to the sandpaper in your fingers. Then, as you progress with that particular grit, you will start to notice that the sandpaper doesn’t bite into the piece like it did at the beginning. As soon as your sandpaper doesn’t feel like it’s doing anything, change up to the next lowest grit that you have available. (You will notice that the new paper grips again...)
Rule 1: Go up in smallish stages – not from coarse to fine in one go (tempting though it may seem). In my experience, its fine to go from 240 to 600 to 1000, but it’s not fine to go from 400 to 1200. You can go from 240-400-600-1000-1200-1500 for the full high mirror polish effect. By the time you get to 1500, your piece will feel like something created in a NASA laboratory...
Rule 2: Empty the water from your bowl between the lower grits and the higher ones, because suspended grit and particles in the water will gouge your newly perfected finish. It’s OK to do 400 and 600 in the same bowl, but I’d change the water (and therefore get rid of the gritty sediment) beween 600 and 800 or 1000.
For each grit, the aim is to give each section of the piece a good rub in several different directions. Try not to just sand in one direction as you’ll simply deepen the scratches the previous grit made.
Phew, I made it to 1000. What now?
Again, its up to you. If you carry on going higher, your piece will eventually become satiny and feel amazing. If you then buff the piece, you will get the full deep mirror shine. Or you can stop at 1000 and buff and get a slightly less spectacular shine (you probably won't notice the difference, to be honest).
If you have perfectionist tendencies, you can buy a marvellous product called Micromesh for higher grits. It’s more like fabric than sandpaper and it lasts a long time and is nicer to use, but it isn't really sandpaper any more - its more a part of polishing.
Micromesh starts around the 1000 mark and goes up to 12,000 (yes, that’s 12 thousand!) but as yet, I've never needed to go that high.
Powered Sanding....
If you're lucky enough to have a bench grinder or bench polisher, you've got the option of taking out some of the donkey work of sanding.
A while ago I bought a Foredom bench polisher which I love and couldn't live without.
I use it for the coarser level of sanding, using nylon jeweller's finishing wheels that I get from www.polishingjewellery.co.uk.
I also buy 'goblet' and 'cylinder' Silverline sanding wheels from Amazon - they're cheap and you can shape them to fit whatever you're sanding.
